Preparation and sintering behavior of Al2O3-Y2O3/ZrB2 composite ...

Preparation and sintering behavior of Al 2 O 3 -Y 2 O 3 /ZrB 2 composite powders 603
Results and Discussion
Influence of pH on the microstructure of Al(OH) 3 -
Y(OH) 3 /ZrB 2 composite powders
Al(OH) 3 -Y(OH) 3 /ZrB 2 composite powders was synthesized
at particular value of the pH. The configuration
of an original and coated ZrB 2 surface is compared in
Fig. 1. A smooth surface is presented on the original ZrB 2
particles. The ZrB 2 surface is coated with a few floccules
of Al(OH) 3 precipitate at pH = 7. Because the pH satisfies
the production of the Al(OH) 3 precipitates from the
Al(NO 3 ) 3 solution via heterogeneous nucleation, the Al 3+
does not form Al(OH) 3 precipitates completely. However,
a layer of symmetrical floccules with Al(OH) 3 and Y(OH) 3
precipitates is coated on the ZrB 2 surface at pH = 9,
which is about 150 nm thick. The thickness of the coating
layer is decreased at pH = 11. Many small spheroidal masses
remain with the Y(OH) 3 precipitates on the ZrB 2 surfaces.
With an increase in the pH of the ZrB 2 suspension, the
entire process from Al(NO 3 ) 3 and Y(NO 3 ) 3 solution to
Al(OH) 3 and Y(OH) 3 precipitate is shown in four steps: no
deposit, heterogeneous nucleation, homogeneous nucleation,
and a complete deposit [14]. The relationship of the
Al 3+ complex and pH is shown in Fig. 2. After the complete
deposition process, Al(OH) 3 is formed as an AlO 2 soluble
complex in the suspension of ZrB 2 by a continuous
increase of the pH [15]. However, Y(OH) 3 precipitates do
not dissolve in the alkaline solution, which still exist on
the ZrB 2 surface. When the pH is 9, Al(OH) 3 -Y(OH) 3 /ZrB 2
composite powders with a better coating quality are formed.
Charactorization of Al 2 O 3 -Y 2 O 3 /ZrB 2 composite
powders
Phase analysis of ZrB 2 particles is shown in Fig. 3, which
Fig. 1. Topographic characteristics of A1(OH) 3 -Y(OH) 3 /ZrB 2
composite powders under different pH conditions (a-Original, b-
pH = 7, c- pH = 9 and d- pH = 11).
Fig. 2. Relation of Al compounds and pH.
Fig. 3. XRD of ZrB 2 after treating under different conditons (a-
Original ZrB 2 , b-Coated ZrB 2 with A1(OH) 3 -Y(OH) 3 and c-Coated
ZrB 2 with Al 2 O 3 -Y 2 O 3 ).
indicates the main phase is the ZrB 2 phase, but a different
phase is found as the conditions are changed. The
backgrounds of Fig. 3(b) and (c) are higher than for
Fig. 3(a). Because ZrB 2 particles are coated with amorphous
A1(OH) 3 and Y(OH) 3 precipitate, Fig. 3(c) shows the
Al 2 O 3 and Y 2 O 3 phases, because A1(OH) 3 and Y(OH) 3
were dehydrated.
Elemental identification and the shell configuration
of ZrB 2 particle surfaces under different conditions are
shown in Fig. 4. The original ZrB 2 particles only show
Zr and B, but the surface of coated ZrB 2 particles shows
Al and Y in addition to Zr and B. A1(NO 3 ) 3 and Y(NO 3 ) 3
have formed A1(OH) 3 and Y(OH) 3 precipitates at pH = 9
coated on the surface of ZrB 2 particles through the coprecipitation.
The surface of the original ZrB 2 particles
is very smooth (Fig. 4(a)). Coated ZrB 2 particles (dried
at 100 o C) are shown in Fig. 4(b); the surface of ZrB 2
particles is coated with a layer of floccules of A1(OH) 3
and Y(OH) 3 precipitates. Coated ZrB 2 particles (calcined
at 600 o C) are shown in Fig. 4(c); the surfaces of ZrB 2
particles are not smooth and many fine particles are evident.
Fig. 3(c) is the phase analysis of composite particles
calcined at 600 o C, and the background of the phase
picture is high, which means the composite particles
contain A1(OH) 3 and Y(OH) 3 .