Gel-tape-casting of aluminum nitride ceramics
J Adv Ceram
Gel-tape-casting of aluminum nitride ceramics
Qiushun SHANG 0 1
Zhengjuan WANG 1
Jun LI 1
a Hailong ZHANG
Shiwei WANG 1
0 University of Chinese Academy of Sciences , Beijing 100049 , China
1 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences , Shanghai 200050 , China
Aluminum nitride ceramic sheets were formed for the first time by a novel and simple method, namely gel-tape-casting process, where only two additives were used to prepare the slurries. PIBM (a water-soluble copolymer of isobutylene and maleic anhydride) acted as both dispersant and gelling agent, and PEG (polyethylene glycol) acted as plasticizer. Combining the advantages of gel-casting and tape-casting processes, flexible and uniform green tapes were obtained. The rheological properties of AlN slurries were studied to optimize the PIBM content and solids loading. After drying and debinding, the green sheets were sintered at 1840 ℃ in N2 atmosphere. Homogeneous microstructure with average grain size of about 7 μm was observed in the final AlN ceramics. Thermal conductivity of the AlN ceramics was 161 W/(m·K).
AlN ceramic sheets; gel-tape-casting; PIBM; rheological properties; microstructure
With high thermal conductivity , high electrical
resistance, and low dielectric property, aluminum
nitride has been considered as the most promising
material for substrates, especially for high power
circuitry in microelectronics field. Generally, high
quality thin AlN ceramic substrates for electronic
applications are fabricated by tape-casting process .
There are two types of tape-casting process based on
different kinds of solvents, non-aqueous and aqueous
systems [3,4]. For the non-aqueous system, organic
additives are flammable and toxic, which would cause
environmental problems. As more and more attention is
paid on environment protection, researchers are trying
to find ways to replace those organics. As a result, the
aqueous system was developed, which is incombustible,
non-toxic, and environmentally friendly.
Up to now, reports on aqueous tape-casting of AlN
were rare. As hydrolysis would occur when AlN
powder was dispersed in water, surface modification
was needed for AlN powder. In 2004, Luo et al.  used
DP270 as both dispersant and hydrolysis inhibitor to
prepare AlN green sheets by aqueous tape-casting. In
2005, Olhero and Ferreira  prepared water-based
AlN slurries by adding H3PO4 as a protective agent
against hydrolysis. Binders and dispersants are also
needed in aqueous tape-casting, and the addition
contents are always very high, which would also cause
environmental problems in the debinding process.
On the other hand, due to the migration of the
solvents, inhomogeneous microstructure would form
along the thickness of the tape, which makes it
impossible to prepare thick sheets. Thus, a complicated
method combining stack and WIP (warm isostatic
pressing) was used to make thick sheets. Gel-casting is
a near-net shaping method for ceramic forming with
complex shapes , which is suitable for preparing bulk
ceramics without forming inhomogeneous
microstructure. In 2002, Xiang et al.  developed a
novel method called gel-tape-casting to produce
alumina sheets. It combined tape-casting with
gel-casting technology which is based on free radical
polymerization. However, five kinds of additives were
used: dispersant, monomer, cross-linking agent, catalyst,
and initiator. In addition, plasticizer was added to
improve the flexibility of green tapes. Except for the
multiple additives, there was an anti-polymerizing
effect of oxygen, which made the process complicated.
In 2004, a novel gel-tape-casting process using sodium
alginate was reported . Sodium alginate was
dissolved in water at room temperature and formed a
three-dimensional network in the presence of
multivalent cations. Styreneacrylic latex was used
to improve the flexibility of green tapes. Gelation
rate could be controlled by the amounts of adipic acid
and sequestrant. Green tapes with homogeneous
microstructure were fabricated by this process. In 2014,
a new system in shaping techniques by in situ
polymerization was reported. Instead of commonly
applied 2-hydroxyethyl acrylate or acrylamide
(monomer) and N,N,N,N-tetramethylethylenedia
mine (activator), 3-O-acryloyl-D-glucose and
Lascorbic acid were used as monomer and activator,
respectively . The new system based on non-toxic
glucose derivatives has been described in detail, and it
allowed to form defect free tapes and dense sintered
bodies by gel-tape-casting.
In 2013, Yang et al.  developed a simple method
for gel-casting of alumina ceramics using a non-toxic
and water-soluble copolymer of isobutylene and maleic
anhydride (simply noted as PIBM). It only needed small
amount of PIBM (0.3 wt%) which acted as both
dispersant and gelling agent for gel-casting of ceramics.
Also, oxygen inhibition would not occur in this system.
Then in 2014, Shu et al.  fabricated AlN ceramics
with a fin-like shape by gel-casting process with
addition of PIBM. These studies show that PIBM would
be a better choice for preparing slurries, which make it
easier for fabricating complicated and large-size
In this study, tape-casting and the simple novel
gel-casting process using PIBM were combined to
obtain flexible and uniform aluminum nitride ceramic
green tapes. Only small amounts of additives were
added. Finally, dense AlN ceramic sheets were
fabricated by pressureless sintering at 1840 ℃ for 4 h in
Commercial AlN powder (Grand F, Tokuyama Co.,
Japan) with an average particle size of 1.1 μm was used
as raw material, and the density was 3.26 g/cm3. Y2O3
was used as sintering aid , and its average particle
size and density was 2.0 μm and 5.01 g/cm3,
respectively. Because of the hydrolysis of these two
kinds of powder , they were coated with a
waterproof agent. The waterproof agent is a commercial
waterproof material, whose main component is
polyurethane. AlN, Y2O3, and the waterproof agent
were mixed in alcohol for 2 h, and then the mixed slurry
was dried at 50 ℃. PIBM with an average molecular
weight of 160 000–170 000 (ISOBAM 110#, Kuraray,
Osaka, Japan) was used as both dispersant and gelling
agent , and its molecular structure is shown in Fig. 1.
Polyethylene glycol (PEG600) was used as plasticizer.
The experimental process of gel-tape-casting is
shown in Fig. 2. First, PIBM solution was prepared, and
the coated AlN and Y2O3 (3 wt% with respect to AlN)
Fig. 1 Molecular structure of PIBM.
Fig. 2 Flowchart of the gel-tape-casting process.
powder and PEG600 (5 wt% with respect to AlN) were
added into the solution to make the slurry. Then, ball
milling was used (200 rpm, 4 h) to make the slurry
well-distributed. The slurry was degassed (1000 rpm,
0.05 atm, 30 s) by THINKY MIXER ARV0-310. Then,
the slurry was poured into the reservoir and tape-casted
in air with a doctor blade of 20 cm in width to get
the green tapes. After demolding and debinding, the
green sheets were sintered at 1840 ℃ for 4 h in N2
Rheological behavior was characterized with a
stress-controlled rheometer (Physica MCR301, Anton
Paar, Graz, Austria) with a parallel plate (25 mm in
diameter). The viscosities of the slurries were measured
with the continuous shear mode increasing from 1 to
1000 s1 at 25 ℃. Microstructure and energy dispersive
spectrum (EDS) test were carried out by scanning
electron microscopy (SEM, JSM-6390, JEOL, Japan)
and field emission scanning electron microscopy
(FESEM, GeminiSEM 500, Zeiss, Germany). Thermal
diffusivity was measured by laser-flash method. The
density of AlN ceramics was measured by the
Archimedes method in distilled water.
Results and discussion
Rheological properties of AlN slurries
High solids loading, good fluidity, and stability are
expected for both tape-casting and gel-casting, so they
are also important in the slurries for gel-tape-casting.
For typical slurries, the rheological behavior is
influenced by ceramic powder, solvent, solids loading,
dispersant, binder, and other organic additives, and the
content of binder is the most important factor . PIBM
is a copolymer of isobutylene and maleic anhydride, so
it can act as dispersant as well as binder in slurries.
Rheological behaviors of the slurries with different
binder content and solids loading were investigated.
3. 1. 1 Viscosity of AlN slurries varying with binder
Rheological properties of the slurries with different
PIBM concentration are shown in Fig. 3. The solids
loading of the slurries was fixed to 38 vol% and the
PIBM addition was in the range of 2–4 wt%
(powder-based) at room temperature. The slurry
compositions are listed in Table 1. It is clear that all
slurries show a shear-thinning character with increasing
Fig. 3 Viscosity of AlN slurries with different PIBM
content (solids loading: 38 vol%).
shear rate. In addition, the viscosity increases with
increasing PIBM content from 2 to 4 wt%. When the
addition content of PIBM increases from 3 to 4 wt%,
there is little change in the viscosity of the slurries.
While, for lower content (2 wt%), the strength of green
tape is poor and it is difficult to get an intact green tape
off the carrier. Therefore, 3 wt% PIBM content was
chosen for the gel-tape-casting and demolding process.
3. 1. 2 Viscosity of AlN slurries varying with solids
Figure 4 shows the viscosity of slurries as a function of
solids loading and 3 wt% PIBM. The compositions of
the slurries are listed in Table 1, and the solids loading is
varied from 38 to 42 vol%. It could be found that all the
slurries show a shear-thinning behavior. The two curves
of slurries with solids loading of 38 and 40 vol% are
very close. And the viscosity increases significantly
when the solids loading increases up to 42 vol%. It is
important to obtain stable and homogeneous slurry with
low viscosity and high solids loading, whereas higher
solids loading can result in high viscosity and
inhomogeneous slurry. As we can see, the slurry with a
solids loading of 40 vol% has proper viscosity and
relatively high solids loading. From Fig. 3 and Fig. 4,
the slurry with 40 vol% solids loading and 3 wt% PIBM
has the best performance, so they are the optimal
conditions for the gel-tape-casting process.
Fig. 4 Viscosity of AlN slurries with different solids
loading (PIBM addition: 3 wt%).
Effect of plasticizer on storage modulus
In traditional gel-casting system, there is no plasticizer.
Thus, after casting and drying, the green body is hard
and brittle. In the gel-tape-casting process, in order to
obtain flexible green tape, plasticizer was used. The
storage modulus is often used for the characterization of
gelation rate. As shown in Fig. 5, without the plasticizer,
the slurry shows a high storage modulus at the start
point and an increased storage modulus behavior with
time going, which is similar to the normal gel-casting
process. After the addition of plasticizer, the curve
shows a similar tendency, but the value of storage
modulus is much smaller. In other words, the plasticizer
greatly prolongs the curing time, and after drying, the
green body is flexible instead of hard and brittle.
Characterization of AlN green tapes
With the addition of appropriate amount of plasticizer,
flexible tapes were obtained and shown in Fig. 6(a), and
the thickness of the tapes was between 250 and 550 μm.
The green tapes produced from slurry with solids
loading of 40 vol% and binder addition of 3 wt% were
Fig. 5 Effect of plasticizer on the evolution of storage
modulus G′ of slurries with a solids loading of 40 vol%.
Fig. 6 (a) Photograph and (b) microstructure of AlN green
tape after casting in air at room temperature.
smooth, uniform, and without cracks. The
microstructure of the green tapes is shown in Fig. 6(b).
It can be seen that the AlN particles are homogeneously
distributed and there are no obvious defects. All the
organic additives were burned out by the debinding
process, with temperature up to 500 ℃.
Properties of dense AlN ceramics
Dense AlN ceramics were obtained by pressureless
sintering at 1840 ℃ for 4 h in N2, as shown in Fig. 7.
The average grain size is about 7 μm, and no visible
pores or defects are observed. However, second phases
can be clearly seen on the grain boundaries. EDS test
was carried out to determine the composition of the
second phases. Figure 8 shows the EDS mapping results
of the fracture surface of the ceramics. Obviously, Y and
O elements are distributed on the grain boundaries. As
the sintering temperature is high, Y2O3 and AlN can
react with each other, so compounds with Y, Al, O, and
Y2O3 can be the second phases. Spot scanning of the
second phases also shows that Al, Y, and O elements are
distributed. The composition of the second phases and
their distribution would affect the thermal conductivity
of AlN ceramics .
The density of the AlN ceramics was determined by
the Archimedes method, which is 3.3 g/cm3. Thermal
diffusivity was measured by the laser-flash method. The
Fig. 7 Microstructure of the dense AlN ceramic fracture
(backscattered electron image).
ceramics is affected by the low thermal conductive
second phases segregated in grain boundary, including
the morphology and microstructure of the second
phases . From the microstructure and EDS analysis
(Figs. 7 and 8), the second phases are connected to each
other, which would decrease the thermal conductivity of
the AlN sheet to a large extent. By changing the
morphology of the second phases into isolated
structures, it can improve the thermal conductivity. On
the other hand, as the distribution of the second phases
is not uniform in the ceramics, it can be inferred that the
waterproof agent does not play an effective role. This
would lead to inhomogeneous hydrolysis of AlN and
Y2O3 in the slurries and increase the generation of
second phases. Further study will be carried out to solve
these problems and improve the thermal conductivity of
the AlN ceramic sheet.
A simple gel-tape-casting method was used to prepare
AlN ceramic sheets. A copolymer PIBM was employed
as both dispersant and gelling agent, and PEG was used
as plasticizer. With the addition of 3 wt% PIBM and a
solids loading of 40 vol%, the aqueous slurries showed
the best performance and flexible green tapes were
obtained. After the debinding and pressureless sintering,
dense AlN sheets with an average grain size of about
7 μm and no big pores or other defects were obtained.
Thermal conductivity of the prepared AlN sheets
reached 161 W/(m∙K). The results showed that PIBM is
an effective and environmentally friendly additive for
gel-tape-casting of AlN ceramic sheets, and the
effective control of the composition and distribution of
the second phases will improve the properties of the
thermal conductivity of the ceramics is determined from
the following formula:
where is the thermal diffusivity (cm2/s), is the
density (g/cm3), and Cp is the heat capacity of the
material (J/(g∙K)). The tested thermal diffusivity is
0.66284 cm2/s. The heat capacity of pure, dense AlN
(0.734 J/(g∙K) at 20 ℃) was used to calculate thermal
conductivity of the AlN ceramics and it is 161 W/(m∙K).
It is lower than the reported one (204 W/(m∙K) in Ref.
). As we know, the thermal conductivity of AlN
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