Effect of polyethylenimine on hydrolysis and dispersion properties of aqueous Si3N4 suspensions

Article abstract of DOI:10.1111/j.1551-2916.2007.01491.x

The roles of polyethylenimine (PEI) in the hydrolysis and dispersion properties of aqueous Si₃-N₄ suspensions were studied in terms of the hydrolysis, adsorption, electrokinetic, and rheological measurements. It was found that the pH change of the suspensions in the acidic environment could be minimized in the presence of ≥ 0.5 dwb% PEI. The ammonia and oxygen measurements suggest that this phenomenon is primarily attributed to the buffer mechanism generated by the ionized PEI, instead of the protection mechanism. The constant pH enables the suspensions to retain a better stability with time at acidic pH. The adsorption of PEI on Si₃N₄ is a high-affinity type at highly basic pH, but is a low-affinity type at acidic pH. As the PEI amount increases, the adsorption shifts the isoelectric point (IEP) of Si₃N₄ from pH 5.9 to pH ～11 until complete coverage is attained. The stability of Si₃N₄ suspensions is found to depend strongly on the saturated adsorption of PEI, which is as a function of the pH and PEI amount. Once the saturated adsorption limit is reached, the excess free PEI molecules become more detrimental to the stability with increased solid loading. The stabilization mechanisms of Si ₃N₄ suspensions by PEI were discussed in detail.

Full text of DOI:10.1111/j.1551-2916.2007.01491.x

J. Am. Ceram. Soc., 90 [3] 797–804 (2007)
DOI: 10.1111/j.1551-2916.2007.01491.x
r 2007 The American Ceramic Society
ournal
J
Effect of Polyethylenimine on Hydrolysis and Dispersion Properties of
Aqueous Si3N4 Suspensions
Xinwen Zhu, Tetsuo Uchikoshi, Tohru S. Suzuki,* and Yoshio Sakka*,w
Fine Particle Processing Group, Nanoceramics Center, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba,
Ibaraki 305-0047, Japan
The roles of polyethylenimine (PEI) in the hydrolysis and dis-
persion properties of aqueous Si3N4 suspensions were studied in
terms of the hydrolysis, adsorption, electrokinetic, and rheolog-
ical measurements. It was found that the pH change of the sus-
pensions in the acidic environment could be minimized in the
presence of ꢀ 0.5 dwb% PEI. The ammonia and oxygen meas-
urements suggest that this phenomenon is primarily attributed to
the buffer mechanism generated by the ionized PEI, instead of
the protection mechanism. The constant pH enables the suspen-
sions to retain a better stability with time at acidic pH. The
adsorption of PEI on Si3N4 is a high-affinity type at highly basic
pH, but is a low-affinity type at acidic pH. As the PEI amount
increases, the adsorption shifts the isoelectric point (IEP) of
Si3N4 from pH 5.9 to pH B11 until complete coverage is at-
tained. The stability of Si3N4 suspensions is found to depend
strongly on the saturated adsorption of PEI, which is as a func-
tion of the pH and PEI amount. Once the saturated adsorption
limit is reached, the excess free PEI molecules become more
detrimental to the stability with increased solid loading. The
stabilization mechanisms of Si3N4 suspensions by PEI were dis-
cussed in detail.
the electrostatic interactions rather than electrosteric interac-
tions.9–11 In general, the effectiveness of polymer dispersants is
related to the Lewis acid–base character of the powders. It is
generally accepted that that the isoelectric point (IEP) of Si3N4
powders is determined by the relative site density of acidic si-
lanol (Si–OH) and basic amine (Si2–NH) groups present mainly
on the surface of Si3N4 powders.12 The presence of a large num-
ber of silanol groups shifts the IEP to lower pH values, whereas
a large number of amine groups shifts the IEP to high values.
The IEP of commercial Si3N4 powders was reported to vary
from pH ꢁ 3 to 9, depending on the powder origin,6–9,12,13
hydrolysis,12,13 and oxidation.6,14 A commercial Si3N4 powder
(UBE SN-E10), one of the most widely used Si3N4 powders, was
reported to be mildly acidic (IEPo7).9,11–13 This means that the
cationic polymers may be more effective than the anionic ones in
stabilizing the aqueous suspensions of this powder.
As a cationic polyelectrolyte, polyethylenimine (PEI) has
been widely used to stabilize aqueous suspensions of various
ceramic powders, such as SiO2,15,16 ZrO2,17,18 TiO2,19 ZnO,20
Y2O3,21 SiC,15,22–26 TiC,27 and TiN,28 similar to the anionic
PAA. PEI is a highly branched macromolecule with a general
chemical formula of [–CH2–CH2–NH–]n.29 The unionized or
partially ionized PEI is readily adsorbed to the negatively
charged surface and thus provides an ‘‘electrosteric’’ or ‘‘steric’’
stabilization for ceramic suspensions. Zhang et al.30 firstly made
an attempt to use PEI as a dispersant to stabilize the aqueous
Si3N4 suspensions. They reported that in the presence of PEI,
the optimum pH is not at the acidic and neutral pH regions, but
at the basic region of 4pH 9 and even near the pHꢁ10.8, where
PEI is essentially uncharged. However, a full understanding of
the role of PEI in the aqueous Si3N4 dispersion is difficult to
establish because of the limited experimental data.
In addition, Si3N4 powder tends to hydrolyze in water, re-
sulting in the time-dependent behavior of the suspensions.13,31,32
Unfortunately, less attention has been paid to the protection of
Si3N4 powder against hydrolysis in aqueous media, not as the
case of AlN powder.33–35 This could be due to three major facts:
(i) the hydrolysis of Si3N4 is not as intense as that of AlN; (ii) the
Si3N4 powder is usually dispersed at the basic pH range, where
the formation and dissolution of the oxide layer (SiO2) enhance
the suspension stability because of the formation of a more neg-
atively charged surface; and (iii) the polymer dispersants (e.g.
PAA) probably play an important role in suppressing simultan-
eously the hydrolysis of Si3N4, to some extent, similar to the case
of AlN.33–35 Despite this, the control of hydrolysis is necessary
for improving the stability of aqueous Si3N4 suspensions. One
effective method is to form a thicker layer of SiO2 on the Si3N4
particle by pre-oxidation treatment.6,14,36 Moreno et al.8 report-
ed that tetramethylammonium hydroxide (TMAH) as a dispers-
ant can provide a protective layer against hydrolysis for Si3N4
powder by adsorption. Therefore, we guess that as a polymer
dispersant, PEI may play a positive role in protecting Si3N4
powder against hydrolysis in water.
I. Introduction
OLLOIDAL processing has received much interest in produ-
cing advanced Si3N4 ceramics because of the potential ca-
C
pacities of minimizing strength-degrading defects, forming
complex shapes, and tailoring microstructure, compared with
the conventional dry-powder pressing method.1–3 For example,
the production of highly textured ceramics can be readily ac-
complished by orientating ceramic particles through tape casting
of large seed particles-containing slurry4 or through slurry con-
solidation in a strong magnetic field.5 In colloidal processing, the
dispersion and stability of the suspensions are of particular im-
portance in controlling the quality of the final products. Owing
to the safety, economic, and environment considerations, much
attention has been focused on the aqueous Si3N4 processing.6–11
The most common polyelectrolytes used for dispersing aque-
ous Si3N4 suspensions are acrylic-based polymers, such as
poly(acrylic acid) (PAA) or its ammonium salt (PAA-NH4),
known as anionic polyelectrolytes.6–11 These studies showed that
better stabilization of Si3N4 suspensions by PAA occurs in the
alkaline region of pH49. As PAA is fully ionized and remains
in the solution as free polymers with a stretched configuration at
pH49, the stabilization is widely believed to be dominated by
V. Hackley—contributing editor
Manuscript No. 21674. Received April 7, 2006; approved October 31, 2006.
This study was supported in part by the Budget for Nuclear Research and a Grant-in-
Aid for Scientific Research of the Japanese Ministry of Education, Culture, Sports, Science
and Technology.
The objective of this work is to carry out a systematic inves-
tigation of the aqueous dispersion of Si3N4 powder with PEI
dispersant. The contents involved in the present work include:
*Member, American Ceramic Society.
wAuthor to whom correspondence should be addressed. e-mail: sakka.yoshio@nims.
go.jp
797

798
Journal of the American Ceramic Society—Zhu et al.
(4) Adsorption Measurements
Vol. 90, No. 3
(1) the effect of PEI on the hydrolysis of Si3N4 in water was
studied at the acidic and basic environments; (2) adsorption be-
havior of PEI on Si3N4 was characterized to understand and
establish the stabilization mechanisms; (3) electrokinetic behav-
ior of Si3N4 versus the pH and PEI amount was characterized by
electroacoustic measurements on the concentrated suspensions;
and (4) the stability of Si3N4 suspensions was evaluated by rhe-
ological measurements as a function of the pH, PEI amount,
solid loading, and stirring time (hydrolysis effect). Our goal is to
provide a better understanding of the role of PEI in the aqueous
Si3N4 dispersion in this study.
The adsorption of PEI on Si3N4 was determined by thermo-
gravimetric analysis (Thermoplus TG 8120, Rigaku Co., Tokyo,
Japan). The 5 vol% Si3N4 suspensions with various amounts of
PEI were prepared by the procedure described above. The sus-
pension pH was adjusted to pH 4 and 10 using 1 N KOH and 1
N HCl, respectively. Then, the suspensions were kept continu-
ously stirred for 4 h to reach equilibrium. Then, the suspensions
were centrifuged at 5000 rpm for 30 min. As a reference, the
sediments from the suspensions without PEI at pH 4 and 10
were also prepared using the same procedure. After decanting
the supernatant, the sediments were dried in an oven at 501C for
24 h, and then at 1101C for 24 h. Figure 1 gives the thermo-
gravimetric analysis curves of pure PEI and a typical Si3N4
powder adsorbed by PEI at a heating rate of 101C in air. The
decomposition of pure PEI occurs mainly between 2501 and
3501C, similar to the report of Wang and Wei.22 Accordingly,
the powder adsorbed by PEI shows a clear weight loss between
2001 and 5001C, which was used to determine the adsorbed
amount of PEI.
II. Experimental Procedure
(1) Materials
A commercial a-Si3N4 powder (SN E10, UBE Industries, Tok-
yo, Japan) was used in this study. As the manufacturer claimed,
this powder had a specific surface area of 11.1 m2/g and an
oxygen content of 1.27 wt%, and major metallic impurities
(like Al, Fe, Ca) with levels of o100 ppm. According to the
electroacoustic measurements, which will be described later, the
Si3N4 powder had a minimum average particle size of 0.51 mm in
suspension. PEI, a highly branched macromolecule with an
average molecular weight of 10000 (Wako Pure Chemical
Industries Ltd., Tokyo, Japan), was chosen as a dispersant.
The distilled water was used as the dispersing medium.
(5) f-Potential Measurements
The z-potential measurements were performed for the 5 vol%
Si3N4 suspensions without and with the desired amounts of PEI,
which were prepared by the procedure described above, using an
acoustic & electroacoustic spectrometer (Model DT-1200,
Dispersion Technology Inc., Bedford Hills, NY). No back-
ground electrolyte solution was used, e.g. KCl, NaCl, etc. The
z potentials were measured as a function of pH by the titration
of a single suspension because of the advantages of the least
amount of time and material required.37 Note that all 5 vol%
Si3N4 suspensions with and without PEI showed initial pH
values 48. As the PEI amount increases to 1.5 dwb%, the pH
increases from 8.3 to 10.3. Thus, the titration was automatically
performed with a built-in autotitrator from the initially basic pH
to 12 by 1 N KOH, and then, a back titration was performed
from pH 12 to 3 by 1 N HCl. It is worth mentioning that this
titration method has no effect on the IEP of Si3N4, but the z-
potential value from descending pH by back titration of 1 N
HCl is slightly lower than that from initially ascending pH by 1
N KOH from the initial pH to 12. This is due to the fact that the
increased concentration of salt ions (K1 and Clꢂ) causes the
compression of the electrical double layer (EDL), thereby de-
creasing the z-potential values. The z-potential-pH curves pre-
sented in this study were obtained from the measured data by
the back titration from pH 12 to 3. The details of the theory and
applications of this measuring technique have been described in
some review papers.38
(2) Suspension Preparation
The suspensions used for all measurements were prepared at
powder volume percentage. The density of a-Si3N4, 3.19 g/cm3
was used for this calculation. The procedure of suspension prep-
aration was as follows: (i) the 5 wt% stock PEI solution was pre-
mixed with the distilled water in a beaker by magnetic stirring;
(ii) the dry powder was slowly added to the pre-mixed solutions
by stirring; (iii) the suspensions were then ultrasonicated for 5
min to break up the agglomerates, using an ultrasonic horn
(USP-600, Shimadzu Inc., Kyoto, Japan) at an output power of
160 W in an ice bath; and (iv) after ultrasonication, the
suspensions were kept continuously stirred before analysis.
The suspension pH was adjusted using reagent-grade hydro-
chloric acid (HCl) and potassium hydroxide (KOH). Without
particular illustration, the pH adjustments were conducted from
the initial (or native) pH points of the suspensions prepared by
procedures (i), (ii), and (iii), using KOH solution to increase the
pH and HCl solution to decrease the pH in the opposite direc-
tions. The added PEI amount was expressed as the dry weight
percentage of the Si3N4 powder basis (dwb%).
(3) Hydrolysis Test
The hydrolysis tests were performed at two different starting pH
values, pH 4 and 10, using the 5 and 10 vol% suspensions with
various amounts of PEI, prepared by the procedure described
above. Then, the beakers were sealed with parafilms and the
suspensions were kept continuously stirred at a mild speed. The
pH values were monitored at the predetermined time intervals.
Then, the suspensions were centrifuged at 5000 rpm for 30 min.
After decanting the supernatant, the sediments were dried in an
oven at 501C for 24 h, and then at 1101C for 24 h. The oxygen
content in the dried powder was determined by an oxygen/
nitrogen analyzer (Model TC-436, LECO Co., St. Joseph, MI).
Furthermore, the supernatants were filtered through a 0.2 mm
membrane. Then, the amount of ammonia leached into the
solutions was measured using a pH-ion meter equipped with an
ammonium ion selective electrode (Model 5002-10C, HORIBA
Ltd., Kyoto, Japan). As the electrode is sensitive only to free
ammonia, the pH of each sample was adjusted to pH 4 12 using
10 N NaOH before measurement. This converts all NH14 into
the free NH3 form. Three different concentrations of standard
NH4Cl solutions, 1, 10, and 100 mg/L, were used to calibrate the
electrode.
0
–20
–40
–60
–80
Pure PEI
–100
0
0
100
200
200
300
300
400
400
500
500
600
600
0.0
–0.2
–0.4
–0.6
–0.8
–1.0
Si3N4 - PEI
100
Temperature (°C)
Fig. 1. Thermogravimetric (TG) curves of pure PEI and a typical Si3N4
powder adsorbed by polyethylenimine (PEI) (Si3N4–PEI) at a heating
rate of 101C/min in air.

March 2007
Effect of Polyethylenimine on Aqueous Si3N4 Suspensions
799
(6) Rheological Measurements
11
The rheological measurements of the suspensions were conduct-
ed with a cone-type viscometer (RC-500, Toki Sangyo Co. Ltd.,
Tokyo, Japan) at a constant temperature of 251C. Considering
the fact that in the presence of PEI, the initial suspensions with
solid loadings of 10 and 30 vol% have highly basic pH values of
49.5, and this value moves to more highly basic pH range
(B11) with increasing PEI amounts, all viscosity measurements
as a function of pH were conducted only by descending pH via
acid titration (HCl). All the initial suspensions were prepared by
means of the procedure described above. Each suspension with a
conditioned pH point was obtained by stirring for 20 min. All
measurements were performed immediately when the suspen-
sions were conditionally prepared.
10
9
PEI (dwb%)
0
8
0.05
0.5
7
6
5
4
3
0
10
20
30
40
50
III. Results and Discussion
Stirring time (h)
(1) Hydrolysis Behavior
Fig. 2. pH versus stirring time for 5 vol% Si3N4 suspensions with
various amounts of polyethylenimine (PEI) at the initial pH 4 and 10,
representing the acidic and basic environments, respectively.
It is recognized that the hydrolysis of Si3N4 powder in water
occurs through the following reaction:
Si3N4 þ 6H2O , 3SiO2 þ 4NH3
(1)
vided by reactions (2) and (4), which makes the final pH remain
in the range of pH 5 9–10.
The release and dissolution of NH3 lead to an increase in
suspension pH as follows:
Figure 3 further gives the effect of stirring time on the pH of
the 10 vol% Si3N4 suspensions at the initial pH 4. It is very clear
that when the PEI amount reaches 0.5 dwb% and above, the
suspensions show a very small increase in pH, less than one pH
unit within the stirring time of 75 h. The present result reveals
that the presence of PEI does result in the stabilization of the pH
of aqueous Si3N4 suspensions to a larger extent whether at the
acidic region or at the alkaline region, particularly in the acidic
region. One possible mechanism for this phenomenon is the
protection mechanism, in which the adsorption of PEI mole-
cules prevents the hydrolysis reaction (1) of Si3N4 by forming a
surface-protective layer, similar to the protection of AlN powder
against hydrolysis by anionic polymers, such as PAA.34,35 Laarz
NH3 þ H2O , NHþ4 þ OHꢂpKa ¼ 9:25
(2)
Moreover, the oxidized surface layer dissolves in the follow-
ing way:
SiO2 þ 2H2O , SiðOHÞ4
(3)
Again, the Si(OH)4 dissociates in the following way:
SiðOHÞ4 , H3SiOꢂ4 þ HþpKa1 ¼ 9:83
SiðOHÞ4 , H2SiO24ꢂ þ 2HþpKa2 ¼ 13:17
(4)
(5)
and Bergstrom also reported that the WC–Co suspensions with
¨
PEI show no change in the pH with time, suggesting a sup-
pressed dissolution of WC.40 The other possible mechanism is
the buffer effect generated by ionized PEI molecules. At
the acidic pH 4, PEI molecule is highly ionized. Thus, the
According to reactions (4) and (5), reaction (3) is catalyzed by
hydroxyl ion (OHꢂ). The previous leaching experiments re-
vealed that the surface dissolution behavior of Si3N4 powder
in water is similar to that of amorphous SiO2 in water.12,13,31,32
The dissolution rate of surface oxidized layer is low in the pH
range of pH o9, but is high in the pH range of pH 49.39
Figure 2 gives the pH change with stirring time for the 5 vol%
Si3N4 suspensions with various amounts of PEI at the initial pH
4 and 10. In the absence of PEI or the presence of PEI of lower
than 0.5 dwb%, the pH shows a rapid increase from the initial
pH 4 to above 9 (B9.2) after stirring for 48 h. This could be
attributed to the release of NH3 species because of the hydrolysis
of Si3N4, according to reactions (1) and (2). However, when the
amount of PEI reaches 0.5 dwb%, the pH shows a very slow
increase, only close to pH 5. In the case of initial pH 10, the
suspension pH change with stirring also depends on the amount
of PEI, although this dependence is not as strong as the case of
the initial pH 4. Without and with 0.05 dwb% PEI, both the
suspensions show a steep decrease from pH 10 to B9.7 within
the initial 4 h. With further increased time, the rate of the de-
crease in pH slows down. After 48 h, the pH reaches B9.5. This
is attributed to the dissociation of the solulable SiO2 (Si(OH)4)
through reaction (4), as is observed by Zhang et al.30 However,
when the PEI amount reaches 0.5 dwb%, the suspensions do not
show a steep decrease in pH within the initial 4 h, which is dif-
ferent from the cases of without and with 0.05 dwb %. Even
with the further stirring, the suspension shows no significant
change in pH. This means that in the basic pH, PEI also allows
the Si3N4 suspensions to retain a more stable pH. Moreover, it is
shown that when no PEI is added, the suspension pH eventually
approaches near pH 9.3 with time whether at an initial pH 4 or
at an initial pH 10. This is attributed to the buffer effect pro-
OHꢂ generated by reactions (1) and (2) can react with the
1
H
released from the ionized PEI by the reaction:
Â
Ã
ꢂCH2 ꢂ CH2 ꢂ NHþ2 , ½ꢂCH2 ꢂ CH2 ꢂ NHꢃnþnHþ
n
(6)
Then, the pH of the system does not change so much. Table I
gives the effect of stirring time on the amount of ammonia
10
9
PEI (dwb%)
0
0.05
8
0.15
0.5
7
0.75
1
6
5
4
3
0
10
20
30
40
50
60
70
80
Stirring time (h)
Fig. 3. pH versus stirring time for 10 vol% Si3N4 suspensions with
various amounts of polyethylenimine (PEI) at the initial pH 4.

800
Journal of the American Ceramic Society—Zhu et al.
Vol. 90, No. 3
100
1.0
0.8
0.6
0.4
0.2
0.0
Table I. Amount of Ammonia (NH3) Released into the 5 vol%
Si3N4 Suspensions without and with 0.5 dwb% PEI as a Func-
tion of Stirring Time at the Initial pH 4
pH 4
pH 10
80
pHw
NH3 amount (mmol/m2)z
100% adsorption
PEI amount (dwb%)
Stirring time (h)
60
40
20
0
0
4
48
4
4.3
9.2
4
2.11
2.21
1.36
2.13
0.5
48
4.7
wpH 5 monitored pH value of suspension after stirring for different time.
zAmount normalized to the specific surface area of Si3N4 powder. PEI, polyethy-
lenimine.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
released into the 5 vol% Si3N4 suspensions without and with 0.5
dwb% PEI at the initial pH 4. It can be found that in both cases,
the amount of NH3 released shows an increase with increased
stirring time. Nevertheless, the amount of NH3 shows a sub-
stantially larger increase with time in the presence of 0.5 dwb%
PEI than in the absence of PEI. This could be associated with
the higher evaporation loss of free NH3 in the latter during stir-
ring because the increased pH converts more NH14 ions into free
NH3. The result indicates that in the presence of 0.5 dwb% PEI,
reaction (1) also proceeds with stirring, compared with the ab-
sence of PEI. Moreover, Table II shows that there is almost no
difference in the oxygen content between the hydrolyzed Si3N4
powders without and with 1 dwb% PEI. As a result, it can be
concluded that the minimized change in the pH of the suspen-
sions, particularly at the acidic pH, is primarily due to the buffer
mechanism provided by PEI, instead of the protection mecha-
nism. This buffer mechanism is achieved when the amount of
PEI reaches up to 0.5 dwb%.
Total PEI amount (dwb%)
Fig. 4. Adsorption isotherms and free PEI percentage of PEI on Si3N4
versus amount of PEI for 5 vol% Si3N4 suspensions at pH 4 and 10.
transition with pH, and the charging character of the powder
surface.
At pH B11, PEI is essentially uncharged (a 5 0) and exists in
a compact coiled conformation because of hydrophobic inter-
actions. The strong nonelectrostatic affinity enables almost all
neutral PEI molecules to adsorb onto the highly negatively
charged particle surface until saturated adsorption is achieved.
This results in the maximum saturated adsorption amount. As
the pH decreases, the ionization (a) of amine groups (–NH–) of
PEI starts to occur by the reaction:
Â
Ã
½ꢂCH2 ꢂ CH2 ꢂ NHꢃnþnHþ , ꢂCH2 ꢂ CH2 ꢂ NH2þ
(7)
n
At pH 10 (aꢁ0.1), PEI molecule is slightly charged and still
retains the coiled form. At the same time, the Si3N4 surface is
still highly negatively charged. Owing to the surface-segment
electrostatic attraction, the adsorption is still high. As the pH
further decreases, the increased charge density of PEI segments
makes the segment–segments repel each other, eventually result-
ing in a conformational transition from the coiled (ao0.4 and
pH48) to the stretched state (a40.4 and pHo8).25,29 When the
pH decreases to 4 (aꢁ0.8), the adsorption significantly de-
creases because of the strong repulsion between the highly posi-
tively charged PEI segments and positively charged powder
surface. Once PEI is fully ionized (a 5 1) at pH 5 B2,25 the ad-
sorption becomes negligible. In addition, the hydrogen bonding
probably occurs between the –NH– groups of PEI and Si–OH
groups of Si3N4 in the whole pH range examined.23 In the high
pH range of pH ꢀ 10, hydrogen bonding occurs between the
dissociated surface silanol (Si–Oꢂ) and the –NH– groups, but in
the pH range of pH o10, hydrogen bonding also occurs
between the undissociated silanol (Si–OH) and the ionized
-NH12 - groups. This is probably different from the adsorption
of PAA on Si3N4.9,11
(2) Adsorption of PEI
Figure 4 gives the adsorption behavior of PEI on the surface of
Si3N4 particles at pH 4 and 10. In the figure, the dotted line
represents complete adsorption of PEI molecules on the solid
surface, and free PEI percentage represents the percentage of the
nonadsorbed PEI present in solution based on the total amount
of PEI added. It is clear that there is a significant difference in
the adsorption behavior between pH 4 and 10. At pH 10, as the
PEI amount increases, most of the PEI molecules are adsorbed
onto the solid surface before a saturated adsorption is attained;
particularly, almost 100% PEI are adsorbed to the solid surface
at a low PEI amount (o0.5 dwb%), suggesting a high-affinity
adsorption type. Once the adsorption exceeds the saturation
limit, the nonadsorbed PEI amount significantly increases.
However, at pH 4, most of PEI molecules are not adsorbed to
the solid surface, but remain as free polymer in the solution,
suggesting a low-affinity adsorption type. At pH 4, the saturated
adsorption amount is approximately 0.09 mg/m2, corresponding
to the added PEI amount of about 0.5 dwb%. But the saturated
adsorption amount is approximately 0.65 mg/m2 at pH 10, cor-
responding to the added PEI amount of 1.5 dwb%. This means
that the saturated adsorption decreases as the pH decreases.
This is similar to the adsorption of PEI on SiO2,29 SiC,23,24,26
ZrO2,17,18 and TiO2.19 This adsorption behavior is related to the
segment charge density of PEI molecule and its conformational
(3) Electrokinetic Behavior
Figure 5 shows the z-potential–pH curves of 5 vol% Si3N4 sus-
pensions with various amounts of PEI. In the absence of PEI,
the native IEP of Si3N4 is B5.9, which is very similar to that of
the Si N powder reported by Laarz and Bergstrom (5.8).11 This
¨
3
4
value is also close to other reported values (6.270.2) for this
powder by other authors.9,12,13 The adsorption of PEI shifts the
IEP from pH 5 5.9 to higher pH values. At 5.9opHo11, the
sign of z potential is reversed due to the specific adsorption of
PEI on the surface of Si3N4. At 3 opH o5.9, the adsorption of
PEI also contributes to the positive z potential. But, below pH 3,
there is no significant difference in the positive z potential with
various amounts of PEI. This is consistent with the negligible
adsorption of PEI, owing to the enhanced repulsion between the
highly positively charged Si3N4 surface and highly ionized PEI
Table II. Oxygen Content of Si3N4 Powder Treated Under
Different Hydrolyzed Conditions
Powder
Oxygen (wt%)
As received
1.24
1.46
1.49
Hydrolyzed without PEIw
Hydrolyzed with 1 dwb% PEIw
wBy stirring 5 vol% Si3N4 suspensions for 48 h at the initial pH 4. PEI, poly-
ethylenimine.

March 2007
Effect of Polyethylenimine on Aqueous Si3N4 Suspensions
801
40
20
PEI (dwb%)
0
0.05
0.15
0.25
0.5
0.75
1
0
−20
−40
1.5
2
2
4
6
8
10
12
pH
Fig. 7. Schematic of polyethylenimine (PEI) interactions at the
solid–liquid interface as a function of pH (in the case of 1.5 dwb%
PEI), showing possible PEI configuration change at the interface, and
potential–distance diagrams indicating surface potential (c0) and shear-
plane potential (cs).
Fig. 5. z-potential versus pH for 5 vol% Si3N4 suspensions with
various amounts of polyethylenimine (PEI).
segments. This is similar to the case of PAA on Si3N4 in the
basic pH range of pH49.9,11
In order to understand the dependence of the PEI amount,
the IEP of Si3N4 as a function of the PEI amount was re-plotted
in Fig. 6. It is clear that the IEP increases rapidly from pH 5.9 to
10.3 as the PEI amount increases to 0.5 dwb%. When the PEI
amount further increases, the IEP increases very slowly and fi-
nally approaches pH B11 at 1.5 dwb% PEI. If the PEI amount
exceeds 1.5 dwb%, the IEP value no longer shows any change.
Several studies revealed that similar to the charged colloidal
particles, the charged PEI also shows an interesting electropho-
retic behavior in water.29,41 They reported that the point of zero
charge (PZC) of PEI is near pH 11. Moreover, Hostetler41 found
that PEI shows a very slight native electrophoretic mobility in
the range of pH411, although PEI contains no functional
group that could give it an anionic charge. The author attrib-
uted the possible reason to the intramolecular segmental asso-
ciation and the presence of unhydrated negative ions in common
solvent cavities. The electrophoretic mobility of polyelectrolytes
in solution has been described qualitatively based on the porous
sphere model (in the case of a coiled polyion) and the line charge
model (in the case of a chain-like polyion).42
where (pHiep SN and (pHiep)PEI are the native IEP values of
)
Si3N4 and PEI, corresponding to 5.9 and 11,29,41 respectively,
and pHiep is the IEP values of Si3N4 in the presence of PEI. As a
result, the coverage, f, can be calculated from the measured
pHiep. The coverage as a function of total PEI amount is also
plotted in Fig. 6. This clearly reveals that the IEP is essentially
associated with the coverage of PEI molecules on the Si3N4 sur-
face. When the PEI amount increases to 1.5 dwb%, the Si3N4
surface achieves a complete coverage. Then, the IEP attains a
native IEP value of PEI, pH B11, where PEI molecules are es-
sentially neutral (a 5 0). The calculated result is in agreement
with the adsorption experiment as discussed above. Previous
studies have shown that the adsorption of PEI shifts the IEP
values of various ceramic powders to near pH 11, regardless of
the acid–base character of the powder itself.15,18–24,26–28,30
A
similar behavior was also reported in the cases of anionic poly-
electrolyes, like PAA.9 These studies suggest that in the presence
of the polymer molecules, the IEP of ceramic powders is con-
trolled by the coverage of polymers on the powder surface.
Furthermore, the possible model of PEI interactions at the
solid–liquid interface as a function of pH is schematically drawn
in Fig. 7, according to the observed adsorption and electroki-
netic results. This is the case of 1.5 dwb% PEI. The adsorption
of PEI may make the hydrodynamic plane of shear move out-
wards from the particle surface. With the decreased pH, the
conformational transition of PEI may shift the plane of shear
further outwards.
Assuming pHiep 5 pHzpc for the Si3N4-PEI systems, thus, the
fraction of surface covered by PEI molecules, f, can be estimated
by the following equation43,44
:
pHiep ꢂ ðpHiepÞSN
f ¼
(8)
ðpHiep
Þ
ꢂ ðpHiepÞSN
PEI
100
80
60
40
20
0
(4) Rheological Behavior
10
(A) Effect of Hydrolysis: Figure 8 shows the pH and
viscosity versus stirring time for 10 vol% Si3N4 suspensions with
0.05 and 1 dwb% PEI at initial pH B3.6. Evidently, at 0.05
dwb% PEI, as the stirring proceeds, the suspension undergoes a
transition process: stabilization (well-dispersed)-destabiliza-
tion (serious aggregation)-restabilization (re-dispersed),
accompanied by a progressive increase in pH from the initial
pH B3.6–9.3. However, at 1 dwb% PEI, the suspension shows
no significant change in both pH and viscosity with stirring,
suggesting a better stability within the stirring time (3 days)
examined.
In order to understand the effect of hydrolysis on the
suspension stability, z-potential measurements as a function of
stirring time for 5 vol% Si3N4 suspensions without and with 1
dwb% PEI at initial pH B3.6 were conducted, as shown in
Fig. 9. In the absence of PEI, as the stirring proceeds, the pH
shifts gradually to higher pH values, whereas the positive z po-
tential decreases. When the pH reaches B7, the z-potential value
IEP
Coverage
8
6
4
2
0
0.0
0.4
0.8
1.2
1.6
2.0
PEI amount (dwb%)
Fig. 6. Effect of polyethylenimine (PEI) amount on the isoelectric point
(IEP) of Si3N4 and its surface coverage (f) by PEI.

802
Journal of the American Ceramic Society—Zhu et al.
Vol. 90, No. 3
(a)
40
30
20
10
0
60
50
40
30
20
10
0
10
−1
Shear rate: 400 s
PEI (dwb%)
0
9
8
7
6
5
4
3
0.05
0.15
0.5
0.75
1
PEI (dwb%)
0.05
1
1.5
3
4
5
6
7
8
9
10
11
0
10
20
30
40
50
60
70
80
pH
Stirring time (h)
Fig. 8. pH and viscosity versus stirring time for 10 vol% Si3N4
suspensions with 0.05 and 1 dwb% polyethylenimine (PEI) at the
initial pH B3.6 (shear rate: 400 sꢂ1).
(b)
PEI (dwb%)
0
0.75
1
1.5
2
1000
100
10
Initial pH
becomes zero. As the pH increases further, the z-potential value
becomes negative. This changed trend in the z potential against
pH is in good agreement with the changed trend in the viscosity
against pH with stirring. In contrast, in the presence of 1 dwb%
PEI, there is almost no change in both the pH and z potential
with stirring. This illustrates that the constant pH is crucial for
retaining the stabilization of the aqueous Si3N4 suspensions. The
increased pH changes the surface chemistry of Si3N4 and thus
destabilizes the suspension at acidic pH. The present result
strongly suggests that the use of PEI is effective for stabilizing
the aqueous Si3N4 suspensions particularly in the acidic pH
range by providing a buffer effect to maintain a stable pH.
(B) Effects of pH, PEI Amount, and Solid Loading: Fig-
ure 10 shows the viscosity versus pH for 10 and 30 vol% Si3N4
suspensions with various amounts of PEI. In the absence of PEI,
the pH dependence of the stability of Si3N4 suspension is inde-
pendent of solid loading, and the suspension reaches a better
stability in the pH range of 49, but drastically aggregated in the
pH range of 4–9, near the IEP. This is in good agreement with
the z-potential curve (see Fig. 5). Although the stabilization can
also be achieved at highly acidic pH o4, this stabilization is
transitional due to the hydrolysis, that is, this stabilization is less
valuable in practice.
−1
Shear rate: 140 s
4
2
6
8
10
12
pH
Fig. 10. Viscosity versus pH for (a) 10 and (b) 30 vol% Si3N4 suspen-
sions with various amounts of polyethylenimine (PEI).
tially the adsorption of PEI. As shown in Fig. 10, one of the
most distinct features is that in the initial states, the addition of a
small amount of PEI causes drastic flocculation for both the 10
and 30 vol% Si3N4 suspensions. The adsorbed polymers induce
the flocculation possibly by the bridging mechanism or surface
charge neutralization.45 It is widely believed that the bridging
flocculation requires very long polymer chains. As PEI shows a
compact coiled conformation before ionization and even at a
low degree of ionization (ao0.4 and pH 4 8),25 the initial floc-
culation could be mainly attributed to the surface charge neu-
tralization instead of the bridging mechanism. However, the PEI
amount increases to 1 and 1.5 dwb%; the 10 and 30 vol% sus-
pensions reach the minimum viscosity, respectively, despite be-
ing close to the IEP (B10.8). This is attributed to the steric
stabilization induced by polymers. The following two major
factors are responsible for the effectiveness of the steric stabil-
ization: (i) the high affinity allows the high adsorption of PEI on
the Si3N4 surface to occur near pH 10.8, and (ii) the PEI adsorbs
at the solid–liquid interface in a compact coiled configuration
and thus forms a dense adsorbed layer, as schematically shown
in Fig. 7. This is consistent with the adsorption behavior. As for
the 10 vol% Si3N4 suspension, the PEI amount for achieving the
minimum viscosity, 1 dwb%, is slightly lower than the adsorp-
tion measured data (B1.5 dwb%). This is not surprising because
the low solid loading is not as sensitive to the complete coverage
as the high solid loading for achieving the stabilization.46–48 The
steric stabilization of PEI near the IEP of pH B10.8 has also
been reported on SiC,24 TiC,27 and Si3N4.30
In the presence of PEI, PEI can either stabilize or flocculate
the suspensions, depending on the PEI amount and pH, essen-
8
40
7
PEI (dwb%)
6
20
0
0
1
5
4
3
−20
120
0
20
40
60
80
100
As shown in Fig. 10, another distinct feature is that the
effect of PEI on the changed trend in the viscosity with pH
depends on the solid loading. At a low solid loading of 10 vol%
(see Fig. 10(a)), as the pH decreases, the viscosity shows an
overall decrease. As the PEI amount increases, the pH range for
Stirring time (h)
Fig. 9. pH and z-potential versus stirring time for 5 vol% Si3N4 sus-
pensions without and with 1 dwb% polyethylenimine (PEI) at the initial
pH B3.6.

March 2007
Effect of Polyethylenimine on Aqueous Si3N4 Suspensions
803
achieving the minimum viscosity moves to higher pH values.
This dependence of the PEI amount against pH has also been
reported for SiO2,29 TiO2,19 SiC,26 and ZrO218 in the literature.
The reason for this is due to the following two major facts: (i)
the decreases in the pH lead to a decrease in the PEI amount
required for achieving the saturated adsorption; (ii) as the pH
decreases, the increased positive charge density of PEI segments
increases and thus provides a higher positive z potential for
particles below the IEP. It could be concluded that the electro-
steric mechanism may be responsible for stabilizing the suspen-
sions with the decreased pH. Moreover, when the PEI amount
increases to 1 dwb% and above, the saturated adsorption pro-
vides a steric stabilization for the suspensions near the IEP. As
the pH decreases, the suspensions still retain better stabilization
even within the whole pH range studied, but the stabilization
mechanism undergoes a transition from the steric to the elec-
trosteric interactions. At the same time, this also indicates that
the excess nonadsorbed PEI has less effect on the stabilization of
the 10 vol% suspensions.
Based on the present study, a stability diagram for aqueous
Si3N4 suspensions with PEI has been proposed, as shown in
Fig. 11. This figure shows that once the saturated adsorption is
reached, the excess free PEI becomes more detrimental to the
stability with increased solid loading. At a high pH range, the
stabilization is dominated by the steric effect, but at pH of o10,
the stabilization may be dominated by the electrosteric effect. In
addition, it is worth mentioning that the stabilization of the
concentrated Si3N4 suspensions with PEI is limited to a narrow
pH range, but the stabilization of dilute Si3N4 suspensions can
be achieved even within the whole pH range (3–11) when the
saturated adsorption is achieved at the starting pH values (no
pH adjustor was used and PEI remains uncharged). The stabil-
ization is also guaranteed by the stabilization in the pH of the
suspensions, particularly in neutral and acidic pH ranges. This
will offer more chance for the wide use of dilute Si3N4 suspen-
sions in some cases, such as aqueous electrophoretic deposition,
in which palladium is preferably used as the cathode substrate to
avoid bubble formation.50
However, at a high solid loading of 30 vol% (see Fig. 10(b)),
when the PEI amount is 0.75 and 1 dwb%, the minimum vis-
cosity reaches pH 8.1 and 9.3, respectively, where the saturated
adsorption may be achieved and thus results in electrosteric sta-
bilization. Note that the viscosity measurements as a function of
pH were not conducted for the cases of a low amount of PEI of
0.75 dwb%, because the 30 vol% Si3N4 suspensions could not
be obtained because of the considerable flocculation. As the pH
decreases further, the viscosity increases, indicating a destabili-
zation. When the PEI amount is 1.5 and 2 dwb%, the minimum
viscosity occurs at pH B10.8 due to steric stabilization. But as
the pH decreases, the viscosity increases, despite the increased
positive z potential (see Fig. 5). This is different from the case of
the low solid loading of 10 vol%. The results indicate that the
stabilization of the concentrated Si3N4 suspensions strongly de-
pends on the optimum pH and the saturated adsorption of PEI.
The reason for this could be due to the flocculation induced by
the excess nonadsorbed PEI molecules,46–48 which accumulates
with the decreased pH because of the desorption from the solid–
liquid interface (see Fig. 7). Also, it is clear that at pH o9.3, the
viscosity generally increases with the increased PEI amount at a
given pH, which is in good agreement with the increased free
PEI concentration. The flocculation induced by the excess free
PEI may be dominated by the depletion mechanism, which is
primarily as a result of an osmotic pressure increase because of
the exclusion of nonadsorbed polymers from the interstices
between particles.47,49 This effect increases with an increase in
the particle volume fraction.
IV. Conclusions
(1) The pH change of aqueous Si3N4 suspensions in
an acidic environment could be minimized by the presence
of ꢀ 0.5 dwb% PEI. This is primarily attributed to the buff-
er mechanism generated by the ionized PEI, instead of the pro-
tection mechanism. The constant pH allows the suspension to
maintain better stability with time at the acidic pH.
(2) The adsorption of PEI on Si3N4 is a high-affinity type at
highly basic pH, but is a low-affinity type at highly acidic pH.
The adsorption decreases with decreased pH. As the PEI
amount increases, the adsorption leads to a shift in the IEP of
Si3N4 from pH 5.9 to pH B11 until complete coverage is
attained.
(3) The dispersion properties of aqueous Si3N4 suspensions
in the presence of PEI are controlled by the PEI amount, pH,
and solid loading. Once the saturated adsorption limit is
reached, the excess free PEI molecules become more detrimen-
tal to the stability of the suspensions with the increased solid
loading.
(4) At the high pH range near IEP (pH B11), the stabil-
ization of the suspensions with PEI is dominated by the steric
effect developed from the neutral PEI, whereas at the pH range
of o10, the stabilization may be dominated by the electrosteric
effect developed from the ionized PEI.
Acknowledgments
The authors would like to thank Prof. Lennart Bergstrom at Stockholm
¨
University for his valuable comments and suggestions. The authors wish to thank
Mr. Nobutaka Oguro at Materials Analysis Station, National Institute for
Materials Science, for his help with the oxygen content measurements.
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