Influence of Sintering Additives on the Porous Structure and Mechanical Properties of Porous Alumina

Journal of Science & Technology 119 (2017) 071-075
71
Influence of Sintering Additives on the Porous Structure
and Mechanical Properties of Porous Alumina
Ảnh hưởng của phụ gia thiêu kết đến cấu trúc và cơ tính của nhôm oxit xốp
Le Minh Hai*, Nguyen Minh Duc, Dang Quoc Khanh
Hanoi University of Science and Technology, No. 1, Dai Co Viet, Hai Ba Trung, Hanoi, Viet Nam
Received: January 10, 2017; accepted: June 9, 2017
Abstract
The aim of the present work is to investigate and to compare the influence of the two sintering additives
Cr2O3 and TiO2 on the porous structure and the mechanical properties of the sintered porous alumina.
Alumina based porous ceramics were fabricated by sacrificial template technique via powder metallurgy
route from alumina powder using ammonium bicarbonate (NH4)HCO3 as the pore former. The results
showed that the use of sintering additives led to a remarkble enhancement of the bonding between alumina
particles hence the strength of the alumina based porous ceramics. Between the two sintering additives,
TiO2 was more effective in aiding sintering (densification) of alumina than Cr2O3. Consequently, the sintered
porous alumina using TiO2 as sintering additive has a smaller pore size, 190 30 m compared to 220 40
m of that with Cr2O3, and lower porosity. The porosity of the sintered samples increased with the
concentration of the pore-forming agents and reaches the highest value of 79.3 % and 82.6% corresponding
to TiO2 and Cr2O3 used. The mechanical strength of the porous alumina varied inversely with porosity.
Keywords: Porous alumina, Titanium dioxide, Chromium oxide, Sintering additive
Tóm tắt
Bài báo trình bày nghiên cứu vai trò và so sánh sự ảnh hưởng của hai loại phụ gia thiêu kết TiO2 và Cr2O3
đến cấu trúc và tính chất cơ học của gốm xốp nhôm oxit sau thiêu kết. Vật liệu gốm xốp nhôm oxit được chế
tạo bằng kỹ thuật mẫu cháy thông qua phương pháp luyện kim bột từ bột nhôm oxit ban đầu sử dụng bột
muối ammoni cacbonat đóng vai trò chất tạo xốp. Kết quả nhận được chứng minh việc sử dụng phụ gia
thiêu kết làm tăng đáng kể liên kết giữa các hạt bột nhôm oxit và qua đó làm tăng mạnh độ bền của mẫu
sau thiêu kết. Trong hai phụ gia thiêu kết sử dụng, TiO2 hiệu quả hơn trong việc tăng khả năng thiêu kết của
vật liệu. Do vậy, mẫu nhôm oxit xốp sử dụng TiO2 làm phụ gia thiêu kết có kích thước lỗ xốp nhỏ hơn
(190 30 m so với 220 40 m của mẫu sử dụng Cr2O3) và có tỷ phần xốp ít hơn. Độ xốp của mẫu sau thiêu
kết tỷ lệ thuận với hàm lượng chất tạo xốp và đạt giá trị cao nhất là 79,3 % và 82,6% tương ứng với phụ gia
thiêu kết TiO2 và Cr2O3. Độ bền của mẫu tỷ lệ nghịch với độ xốp đạt giá trị cao hơn đối với các mẫu sử dụng
phụ gia thiêu kết TiO2.
Từ khóa: nhôm oxit xốp, oxit titan, oxit crom, phụ gia thiêu kết
1. Introduction1
Advanced porous ceramics have been utilized in
a broad range of applications in environmental,
biological, and transportation issues [1]. Ceramic
materials possessed many advantages over other
materials such as polymers or metals. The properties
of hardness, chemical inertness, thermal shock
resistance, corrosion and wear resistance, and low
density are essential to many applications [2-3].
Porous ceramics could be prepared through
many processing techniques divided into three basic
routes: replica, sacrificial template, and direct
foaming [3]. Among the processing techniques,
* Corresponding author: Tel.: (+84) 912.098.484
Email: hai.leminh@hust.edu.vn
sacrificial template method appears to be the most
effective method for processing foams intended for
thermal insulation applications. For highly porous
ceramics, the relatively high mechanical strength
could be attainable by this method with a relatively
well established microstucture. The sacrificial
template processing technique incorporates pore
former to act as a place holder within the ceramic
powder or slurry. Once the green body is formed, the
pore former is removed to leave behind the empty
pores [3]. Powder metallurgy is one of the widely
used methods within sacrificial template route. The
process consists of applying a pressure uniaxially or
isostatically to form a green body. Whatever the
compaction process, the pore former is uniformly
mixed to the starting powder before compaction and
is eliminated during sintering step [4].
Journal of Science & Technology 119 (2017) 071-075
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Recently, alumina is the most commonly used
ceramic material. It posseses high temperature
stability and low thermal conductivity that are
requirements for thermal insulators. Porous alumina
could be processed by a partial sintering method
without pore former. However, partial sintered
products usually possess random microstructure and
low porosity (<30%) [5]. High porosity could be
attained using different pore formers such as wax
spheres, naphthalene particles, PMMA, NaCl,
starch... [6]. The porous fraction must be controlled in
relation to mechanical strength, since these properties
are generally inversely related.
The typical sintering temperature of pure
alumina is approximately 1700°C in normal
laboratory practice [7]. Experimentation have
indicated that the sintering temperature of alumina
can be lowered when small particles were used. On
the other hand, the addition of certain sintering
additives such as CuO2, TiO2, MgO và Cr2O3 could
promote the sintering of alumina at temperatures
below 1700°C [7,8]. These oxides could react with
Al2O3 to form either solid solutions with defect
lattices or intermediate phases that assist diffusional
processes. Moreover, sintering additives not only
enhance the sinterability of alumina but also
significantly increase the mechanical properties of the
sintered products.
In our previous works, the results showed that
the sinterability and the strength of the sintered
alumina were considerably improved using Cr2O3 and
TiO2 as sintering additives with the suitable
concentration of 0.5 and 1.0 wt.%, respectively. In
the present work, highly porous alumina was
prepared via powder metallurgy method using
ammonium bicarbonate (NH4)HCO3 as the pore
former and Cr2O3 or TiO2 as sintering additive. The
aim of our research work is to investigate and to
compare the role of the two sintering additives on the
porous structure and the mechanical properties of the
sintered porous alumina.
2. Experimental procedure
The Al2O3 starting powder (Xilong Ltd., China)
was of 99.5% purity with an average particle size of 8
µm. The sintering additives were chromium oxide
Cr2O3 powder (99.9%) and titanium oxide TiO2
powder (99%) with an average particle size of 4 µm,
purchased from Xilong Ltd., China. Powder mixtures
of 99.5 wt.% Al2O3 + 0.5 wt.% Cr2O3 and 99.0 wt.%
Al2O3 + 1.0 wt.% TiO2 were ball-milled for 24 h
using highly pure Al2O3 balls prepared in a highly
pure ethanol solution and, then dried in an oven at
110oC for 3h. The obtained powder mixtures were
then mixed with the pore former using a dried ball-
mixing for 3h. Ammonia bicarbonate (NH4HCO3)
powder was used as pore formers. In order to
investigate the influence of the pore former
concentration on the porosity and properties of the
sample, the fraction of the pore former was varied
from 30 to 80 vol.%. The green pellets were formed
by uniaxial pressing in a 20 mm diameter cylindrical
steel die with a pressure of 300 MPa.
The green pellets were annealed at 200oC for 2h
to totally eliminate the pore former and at 500oC for
1h to remove the PVA binder. At 200oC, ammonium
bicarbonate (NH4HCO3) was thermally decomposed
according the following reactions:
NH4HCO3 NH3 + CO2 + H2O (1)
Finally, the pellets were sintered in an electrical
resistance heating furnace (HT1600, Linn, Germany)
at 1550ºC for 4 hours in argon atmosphere.
The densities of green pellets were determined
from the weight and volume. Average of the 3 density
measurements were taken into consideration. The
apparent porosity, bulk density of sintered pellets was
measured according to Archimede’s principle. First
the dry weight of pellets was measured. Then they
were soaked in distilled water kept inside a beaker
and were evacuated in a vacuum evacuator till all the
air bubbles vanished. After removing from vacuum
evacuator, the suspended weight and soaked weight
of the samples were determined. Porosity in % of the
sintered pellets were calculated according to the
following formula:
Porosity = (soaked weight - dry weight)*100/(soaked
weight - suspended weight).
The morphology and EDX analysis were carried
out on fracture surface using a scanning electron
microscopy (JOEL). The compressive and the
bending strengths were tested on MTS 300 according
to JIS-R 1608-2003 và JIS-R 1664-2004 standards,
respectively.
3. Results and Discussion
The SEM of the Al2O3 samples sintered at
1550oC for 4h without and with different sintering
additives of 0.5% Cr2O3 or 1.0% TiO2 were shown in
Figure 1. When sintering additve was not used
(Figure 1a), a very poor bonding was observed at the
Al2O3 grain boundary. On the other hand, the particle
bonding was significantly enhanced with the addition
of 0.5% Cr2O3 (Figure 1b) or 1.0% TiO2 (Figure 1c)
as sintering additives, especially, in the case of TiO2.
The microstructure of the sintered samples showed a
good adhesion at the grain boundary. The obtained
results indicated the effectiveness of Cr2O3 and TiO2
in sintering of Al2O3. The improvement of the
sinterability could be explained by the presence of the
intermediate phases on the Al2O3 particle surface
Journal of Science & Technology 119 (2017) 071-075
73
when sintering additives were used. These
intermediate phases were formed from the solid state
reactions between Al2O3 and Cr2O3 or TiO2 during
sintering process. According to the binary phase
diagram of Al2O3/Cr2O3, the oxides possess the same
crystal structure having a hexagonal structure and
therefore, could be reacted to form solid solution (Al2-
x, Crx)O3 in the whole range composition [9]. In the
case of Al2O3-TiO2 system, it is reported [10] that the
solid solubility of TiO2 in Al2O3 is too small. The
solid solution of TiO2 in Al2O3 was found at
>1150oC, and the solubility was 0.27% at 1700oC.
Beyond the solubility limit, excess TiO2 coexisted
with Al2O3 as rutile below 1350oC and as Al2TiO5
above 1450oC. The existences of the intermediate
phases could not be validated using XRD method due
to their low fraction. However, elemental analysis by
energy dispersive X-ray (EDX) revealed the existence
of Cr or Ti on the surface particle. Besides, the
formation of the intermediate phases was also
recognized through the colors of the sintered samples
(Figure 2). The white color of without-sintering-
additive sample turns to pink and grey color
corresponding to the addition of Cr2O3 and TiO2,
respectively. The change of the sample color was
totally in agreement with the references [11-13].
The porosity and the mechanical properties of
the sintered samples without and with sintering
additives were showed in Table 1. The addition of
sintering additives led to a good bonding between the
particles and consequently, enhanced the
densification of the samples after sintering. It could
be attributed to the lower porosity and much higher
compressive and bending strengths of the samples
with sintering additives compared to that of sample
containing Al2O3 alone. The results also prove that
TiO2 is more effective than Cr2O3 in aiding sintering
of Al2O3. The strength of the sample with TiO2 was
superior than that with Cr2O3.
Table 1. Porosity, bending and compressive strengths
of sintered Al2O3 without and with sintering additives
of 0.5% Cr2O3 or 1.0% TiO2
Sample Porosity(%)
Bending
strength
(MPa)
Compressive
strength
(MPa)
Al2O3 39 3 45.2 66.2
Al2O3 +
0.5%
Cr2O3
38 4 82.0 99.6
Al2O3 +
1.0%
TiO2
30 3 128.3 368.5
Fig. 1. SEM images of the Al2O3 based ceramics
sintered at 1550oC for 4h (a) without and with
sintering additives of (b) 0.5% Cr2O3 (c) 1% TiO2
1 m
(a)
1 m
(b)
(c)
2 m
Journal of Science & Technology 119 (2017) 071-075
74
Fig. 2. Images of the Al2O3 based ceramics (a)
without and with sintering additives of (b) Cr2O3 and
(c) TiO2
Fig. 3. SEM images of the porous alumina sintered
sintered at 1550oC for 4h using sintering additives of
(a) Cr2O3 (b) TiO2 with 80 vol% of the pore former
ammonium bicarbonate (NH4)HCO3 80%
SEM images of porous samples fabricated using
80 vol.% of (NH4)HCO3 as the pore former with
different sintering additives were shown in Figure 3.
Whatever the sintering additive has been used, the
samples have irregular pore. However, the average
pore size was change with different sintering
additives. The pore sizes and pore size distribution of
each sample were determined from the corresponding
SEM images using ImageJ software. When TiO2 was
used, samples have the average pore size of 190 30
m which is smaller than that with Cr2O3 showing
220 40 m. As can be seen, the pore shape and size
is different from the particle size and shape of the
starting pore former. NH4HCO3 particles are spherical
with the average size of 120 m. This phenomena
could be performed due to several reasons. They
include the agglomeration of the pore former particles
during powder mixing, the pressure increasing during
pore former removal or the densification of the
sample volume during sintering process.
In order to investigate the influence of the pore
former concentration on the porosity and the strength
of the porous sample, the volume fraction of the pore
former was varied from 30 to 80 %. Figure 4 exhibits
the porosity of the sintered samples with different
sintering additives, Cr2O3 and TiO2, as a function of
the pore former concentration. As can be seen, the
porosity of the sample increases with the pore former
concentration. When the pore former concentration
was increased from 0 to 80 vol.%, the porosity of the
sintered sample using Cr2O3 as sintering additive
increased from 38.0 to 82.6% which is higher than
that using TiO2 going from 28.9 to 79.3%. When the
pore former excess 80 vol.%, the samples were
collapsed after pore former elemination process
whatever sintering additive was used.
Figure 5 shows the compressive strength of the
sintered samples as a function of pore former
concentration. As expected, higher porosity of the
sintered sample leads to a lower compressive
strength. As TiO2 using as sintering additive, when
the porosity increases from 29.8 to 78.3%
corresponding to the pore former concentration going
from 0 to 80%, the compressive strength drastically
decreases from 368.5 to 1.4 MPa. On the other hand,
the compressive strength of the sintered porous
alumina using Cr2O3 as sintering additive also
decreased from 99.6 to 0.9 MPa when the porosity
increased from 38.0 to 82.6%.
Fig. 4. Influence of the pore former concentration on
the porosity of the porous alumina sintered at 1550oC
for 4h using different sintering additives
(a)
(b)
(a) (b) (c)
Journal of Science & Technology 119 (2017) 071-075
75
Fig. 5. Influence of the pore former concentration on
the compressive strength of the porous alumina
sintered at 1550oC for 4h using different sintering
additives
The refractoriness of the porous alumina
samples without and with different sintering additives
were determined by Pyrometrics Cone Equivalent
(PCE) test. PCE is measured by making a cone of the
refractory and firing it until it bends and comparing it
with the standard cone. The result showed that at the
maximum temperature 1770oC of the equipment, no
cones were bended, which indicated that the heat
resistance of the samples are higher than 1770oC.
4. Conclusions
Sintering additives Cr2O3 and TiO2 significantly
enhanced the sinterability of alumina at 1550oC and
improved the mechanical properties of the products,
especially, in the case of TiO2. Porous alumina
prepared by powder metallurgy using NH4HCO3 as
pore former and TiO2 as sintering additive has lower
porosity, smaller pore size and higher strength than
that using Cr2O3. The porosity of the sintered porous
alumina increased with the concentration of the pore
former. However, the strength of the sample changed
inversely with the porosity. The heat resistance of the
sintered porous alumina are higher than 1770oC
whatever sintering additives were used.
Acknowledement
The authors sincerely thank to Dr. DO Minh
Duc for the generous help in testing mechanical
strength. This research is funded by Hanoi University
of Science and Technology (HUST) under project
number T2016-PC-151.
References
[1] E.C. Hammel, O.L.-R. Ighodaro, O.I. Okoli,
Processing and properties of advanced porous
ceramics: An application based review, Ceram. Int.
40 (2014) 15351–15370.
[2] I.Y. Guzman, Certain Principles of Formation of
Porous Ceramic Structures. Properties and
Applications (A Review), Glass Ceram. 60 (2003)
280–283.
[3] D. Li, M. Li, Porous Y2SiO5 Ceramic with Low
Thermal Conductivity, J. Mater. Sci. Technol. 28
(2012) 799–802.
[4] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J.
Gauckler, Processing Routes to Macroporous
Ceramics: A Review, J. Am. Ceram. Soc. 89 (2006)
1771–1789.
[5] E. Chevalier, D. Chulia, C. Pouget, M. Viana,
Fabrication of porous substrates: a review of
processes using pore forming agents in the
biomaterial field, J. Pharm. Sci. 97 (2008) 1135–
1154.
[6] T. Ohji, M. Fukushima, Macro-porous ceramics:
processing and properties, Int. Mater. Rev. 57 (2012)
115–131.
[7] L.A. Xue, I.-W. Chen, Low-Temperature Sintering of
Alumina with Liquid-Forming Additives, J. Am.
Ceram. Soc. 74 (1991) 2011–2013.
[8] I.B. Cutler, C. Bradshaw, C.J. Christensen, E.P.
Hyatt, Sintering of Alumina at Temperatures of
1400°C and Below, J. Am. Ceram. Soc. 40 (1957)
134–139.
[9] Seung-Am Cho, F.J. Arenas, J. Ochoa, Densification
and hardness of Al2O3-Cr2O3 system with and without
Ti addition, Ceram. Int. (1990) 301- 309.
[10] R. D. Bagley, I.B. Cutler and D. L. Johnson, Effect of
TiO2 on Initial Sintering of Al2O3, J. Am. Ceram.
Soc. 53 (1970) 136–141.
[11] J.J. Kanga, B.S.Xub, H.D. Wang, C.B. Wanga,
Influence of Spraying Parameters on the
Microstructure and Properties of Plasma-sprayed
Al2O3/40%TiO2 Coating, Physics Procedia Vol. 50
(2013) 169–176.
[12] K.Q. Dang and M. Nanko, Effects of pressure and
temperature on sintering of Cr-doped Al2O3 by pulsed
electric current sintering process, IOP Conf. Series:
Mater. Sci. and Eng. 20 (2011) 012004
[13] A. Azarniya , A. Azarniya, H. R. Hosseinia,
Abdolreza Simchib, Mechanical and thermal
properties of highly porous Al2TiO5–Mullite
ceramics, Ceram. Int., Vol. 42, Issue 2, Part B (2016)
3548–3555.