Influence of sintering additives on the porous structure and mechanical properties of porous alumina

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Journal of Science & Technology 119 (2017) 071-075

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

Cr₂O₃and TiO₂on the porous structure and the mechanical properties of the sintered porous alumina.

Alumina based porous ceramics were fabricated by sacri cial template technique via powder metallurgy

route from alumina powder using ammonium bicarbonate (NH₄)HCO₃as 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,

TiO₂was more effective in aiding sintering (densification) of alumina than Cr₂O₃. Consequently, the sintered

porous alumina using TiO₂as 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 TiO₂and Cr₂O₃used. The mechanical strength of the porous alumina varied inversely with porosity.

Keywords: Porous alumina, Titanium dioxide, Chromium oxide, Sintering additive

Tó m t t

Bài bá o trì nh bày nghiên c u vai trò và so sá nh s

n c u trúc và tí nh ch t c h c c a g m x p nhô m oxit sau thiêu k t. V t li u g m x p nhô m oxit

t o b ng k thu t m u chá y thô ng qua ph ng phá p luy n kim b t t b t nhô m oxit ban u s d ng b t

mu i ammoni cacbonat ó ng vai trò ch t t o x p. K t qu nh n c ch ng minh vi c s d ng ph gia

b n c a m u

nh h ng c a hai lo i ph gia thiêu k t TiO₂và Cr₂O₃

c ch

thiêu k t làm t ng á ng k liên k t gi a cá c h t b t nhô m oxit và qua ó làm t ng m nh

sau thiêu k t. Trong hai ph gia thiêu k t s d ng, TiO₂hi u qu h n trong vi c t ng kh n ng thiêu k t c a

v t li u. Do v y, m u nhô m oxit x p s d ng TiO₂làm ph gia thiêu k t có kí ch th c l x p nh h n

(19030 m so v i 22040 m c a m u s d ng Cr₂O₃) và có t ph n x p í t h n.

x p c a m u sau thiêu

k t t l thu n v i hàm l ng ch t t o x p và t giá tr cao nh t là 79,3 % và 82,6% t ng ng v i ph gia

thiêu k t TiO₂và Cr₂O₃.

ph gia thiêu k t TiO₂.

b n c a m u t l ngh ch v i

x p t giá tr cao h n i v i cá c m u s d ng

khó a: nhô m oxit x p, oxit titan, oxit crom, ph gia thiêu k t

1. Introduction¹

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].

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: [email protected]

Journal of Science & Technology 119 (2017) 071-075

Recently, alumina is the most commonly used

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.

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

The green pellets were annealed at 200^oC for 2h

to totally eliminate the pore former and at 500^oC for

1h to remove the PVA binder. At 200^oC, ammonium

bicarbonate (NH₄HCO₃) was thermally decomposed

according the following reactions:

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.

NH₄HCO₃ NH₃+ CO₂+ H₂O

(1)

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 Cr₂O₃could

promote the sintering of alumina at temperatures

below 1700°C [7,8]. These oxides could react with

Al₂O₃to 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.

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:

In our previous works, the results showed that

the sinterability and the strength of the sintered

alumina were considerably improved using Cr₂O₃and

TiO₂as 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)HCO₃as the pore

former and Cr₂O₃or TiO₂as 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.

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 Al₂O₃samples sintered at

1550^oC for 4h without and with different sintering

additives of 0.5% Cr₂O₃or 1.0% TiO₂were shown in

Figure 1. When sintering additve was not used

(Figure 1a), a very poor bonding was observed at the

Al₂O₃grain 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 Cr₂O₃and TiO₂

in sintering of Al₂O₃. The improvement of the

sinterability could be explained by the presence of the

intermediate phases on the Al₂O₃particle surface

2. Experimental procedure

The Al₂O₃starting powder (Xilong Ltd., China)

was of 99.5% purity with an average particle size of 8

µm. The sintering additives were chromium oxide

Cr₂O₃powder (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.% Cr₂O₃and 99.0 wt.%

Al₂O₃+ 1.0 wt.% TiO₂were ball-milled for 24 h

using highly pure Al₂O₃balls prepared in a highly

pure ethanol solution and, then dried in an oven at

110^oC for 3h. The obtained powder mixtures were

then mixed with the pore former using a dried ball-

mixing for 3h. Ammonia bicarbonate (NH₄HCO₃)

Journal of Science & Technology 119 (2017) 071-075

when sintering additives were used. These

(a)

intermediate phases were formed from the solid state

reactions between Al₂O₃and Cr₂O₃or TiO₂during

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 (Al_2-

_x, Cr_x)O₃in the whole range composition [9]. In the

case of Al₂O₃-TiO₂system, it is reported [10] that the

solid solubility of TiO₂in Al₂O₃is too small. The

solid solution of TiO₂in Al₂O₃was found at

>1150^oC, and the solubility was 0.27% at 1700^oC.

Beyond the solubility limit, excess TiO₂coexisted

with Al₂O₃as rutile below 1350^oC and as Al₂TiO₅

above 1450^oC. 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 Cr₂O₃and TiO₂,

respectively. The change of the sample color was

totally in agreement with the references [11-13].

2 m

(b)

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

1 m

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 Al₂O₃alone. The results also prove that

TiO₂is more effective than Cr₂O₃in aiding sintering

of Al₂O₃. The strength of the sample with TiO₂was

superior than that with Cr₂O₃.

(c)

Table 1. Porosity, bending and compressive strengths

of sintered Al₂O₃without and with sintering additives

of 0.5% Cr₂O₃or 1.0% TiO₂

1 m

Bending Compressive

Porosity

(%)

Sample

strength

(MPa)

strength

(MPa)

Fig. 1. SEM images of the Al₂O₃based ceramics

sintered at 1550^oC for 4h (a) without and with

sintering additives of (b) 0.5% Cr₂O₃(c) 1% TiO₂

Al₂O₃

45.2

82.0

66.2

99.6

393

384

Al₂O₃+

0.5%

Cr₂O₃

Al₂O₃+

1.0%

128.3

368.5

303

TiO₂

Journal of Science & Technology 119 (2017) 071-075

is different from the particle size and shape of the

starting pore former. NH₄HCO₃particles 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.

(a)

(b)

(c)

Fig. 2. Images of the Al₂O₃based ceramics (a)

without and with sintering additives of (b) Cr₂O₃and

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, Cr₂O₃and 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 Cr₂O₃as sintering additive

increased from 38.0 to 82.6% which is higher than

that using TiO₂going 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.

(a)

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 TiO₂using 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 Cr₂O₃as sintering additive also

decreased from 99.6 to 0.9 MPa when the porosity

increased from 38.0 to 82.6%.

(b)

Fig. 3. SEM images of the porous alumina sintered

sintered at 1550^oC for 4h using sintering additives of

(a) Cr₂O₃(b) TiO₂with 80 vol% of the pore former

ammonium bicarbonate (NH₄)HCO₃80%

SEM images of porous samples fabricated using

80 vol.% of (NH₄)HCO₃as 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 TiO₂was

used, samples have the average pore size of 19030

m which is smaller than that with Cr₂O₃showing

220 40 m. As can be seen, the pore shape and size

Fig. 4. Influence of the pore former concentration on

the porosity of the porous alumina sintered at 1550^oC

for 4h using different sintering additives

Journal of Science & Technology 119 (2017) 071-075

References

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Processing and properties of advanced porous

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Fig. 5. Influence of the pore former concentration on

the compressive strength of the porous alumina

sintered at 1550^oC for 4h using different sintering

additives

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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 1770^oC of the equipment, no

cones were bended, which indicated that the heat

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1400°C and Below, J. Am. Ceram. Soc. 40 (1957)

134–139.

4. Conclusions

Sintering additives Cr₂O₃and TiO₂significantly

enhanced the sinterability of alumina at 1550^oC and

improved the mechanical properties of the products,

especially, in the case of TiO₂. Porous alumina

prepared by powder metallurgy using NH₄HCO₃as

pore former and TiO₂as 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 1770^oC

whatever sintering additives were used.

[9] Seung-Am Cho, F.J. Arenas, J. Ochoa, Densification

and hardness of Al₂O₃-Cr₂O₃system with and without

Ti addition, Ceram. Int. (1990) 301- 309.

[10] R. D. Bagley, I.B. Cutler and D. L. Johnson, Effect of

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Soc. 53 (1970) 136–141.

[11] J.J. Kanga, B.S.Xub, H.D. Wang, C.B. Wanga,

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Microstructure and Properties of Plasma-sprayed

Al2O3/40%TiO2 Coating, Physics Procedia Vol. 50

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temperature on sintering of Cr-doped Al₂O₃by pulsed

electric current sintering process, IOP Conf. Series:

Mater. Sci. and Eng. 20 (2011) 012004

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.

[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.

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