Effect of Cr2O3addition on crystallization, microstructure and properties of Li2O–Al2O3–SiO2glass-ceramics

Article abstract of DOI:10.1016/j.jallcom.2016.09.043

A new glass-ceramic with high flexural strength and low thermal expansion based on Li₂O–Al₂O₃–SiO₂(LAS) was prepared in this study. The effects of Cr₂O₃addition on the crystallization, microstructure, flexural strength, thermal expansion, and electrical properties of LAS system were investigated. The crystallization kinetics based on DSC analysis was calculated using Kissinger and Ozawa methods, which showed that the activation energy E decreases from 158.5 to 149.3?kJ/mol, indicating that Cr₂O₃is beneficial to the crystallization of LAS; the crystallization index n varies between 4.46 and 5.09, indicating that the crystallization manner is the volumetric crystallization. XRD analysis was estimated by the whole pattern fitting method, demonstrating that Cr₂O₃addition could change the phase contents and promote the crystallinity. The crystallization of CaMgSi₂O₆and Cr₂O₃with higher CTE not only properly adjusted CTE for matching Si, but also dramatically improved the flexural strength for LAS glass-ceramic. Moreover, we provided a modified formula to calculate CTE of glass-ceramic in the acceptable range. LAS glass-ceramic with 3?wt% Cr₂O₃sintered at 800?°C exhibited good properties: σ?=?208?MPa, α?=?2.64?×?10^?6/°C, ε?=?8.3, tanδ?=?3.6?×?10^?3, ρ?=?8.82?×?10¹²?Ω?cm, indicating its suitability for LTCC application.

Full text of DOI:10.1016/j.jallcom.2016.09.043

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Journal of Alloys and Compounds 693 (2017) 9e15
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Effect of Cr2O3 addition on crystallization, microstructure and
properties of Li2OeAl2O3eSiO2 glass-ceramics
, b, c,
Bo Li a
*, Shanlin Wang a, Yi Fang a
a School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China
b State Key Laboratory of Electronic Thin Films and Integrated Devices, Chengdu 610054, China
c National Engineering Research Center of Electromagnetic Radiation Control Materials, Chengdu 610054, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new glass-ceramic with high flexural strength and low thermal expansion based on Li2OeAl2O3eSiO2
(LAS) was prepared in this study. The effects of Cr2O3 addition on the crystallization, microstructure,
flexural strength, thermal expansion, and electrical properties of LAS system were investigated. The
crystallization kinetics based on DSC analysis was calculated using Kissinger and Ozawa methods, which
showed that the activation energy E decreases from 158.5 to 149.3 kJ/mol, indicating that Cr2O3 is
beneficial to the crystallization of LAS; the crystallization index n varies between 4.46 and 5.09, indi-
cating that the crystallization manner is the volumetric crystallization. XRD analysis was estimated by
the whole pattern fitting method, demonstrating that Cr2O3 addition could change the phase contents
and promote the crystallinity. The crystallization of CaMgSi2O6 and Cr2O3 with higher CTE not only
properly adjusted CTE for matching Si, but also dramatically improved the flexural strength for LAS glass-
ceramic. Moreover, we provided a modified formula to calculate CTE of glass-ceramic in the acceptable
Received 22 June 2016
Received in revised form
19 August 2016
Accepted 4 September 2016
Available online 6 September 2016
Keywords:
Glass-ceramic
LTCC
Coefficient of thermal expansion
Crystallization kinetics
range. LAS glass-ceramic with 3 wt% Cr2O3 sintered at 800 ꢀC exhibited good properties:
s
¼ 208 MPa,
a
¼ 2.64 ꢁ 10ꢂ6/ꢀC, ε ¼ 8.3, tan
d
¼ 3.6 ꢁ 10ꢂ3
,
r
¼ 8.82 ꢁ 1012
U cm, indicating its suitability for LTCC
application.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
of ZrO2 and TiO2 on microstructures and properties of LAS, and
obtained low CTE of 2.56e2.68 ꢁ 10ꢂ6/ꢀC and high flexural strength
of 151e157 MPa [14,17].
In recent years, low temperature co-fired ceramic (LTCC) has
been extensively employed in microelectronic packages for multi-
layer integrated circuits [1e4]. In order to meet high packing
density and high frequency transportation, LTCC substrate mate-
rials should possess low thermal expansion (<3 ꢁ 10ꢂ6/ꢀC), high
mechanical strength (>150 MPa), and low dielectric constant (<9)
[5e8].
Many glass-ceramic systems, such as Li2OeAl2O3eSiO2 [9],
MgOeAl2O3eSiO2 [10], CaOeAl2O3eSiO2 [11], and CaOeB2O3eSiO2
[12] have been developed. Further, the nucleating agents ZrO2, TiO2,
and P2O5 and their effects have been reported [13e15]. Among
these systems, Li2OeAl2O3eSiO2 glass-ceramic, due to its low co-
efficient of thermal expansion (CTE) [16], could match silicon wafer
well and has attracted our attention. We thus studied the influence
Some researchers investigated the nucleation effect of Cr2O3 in
other systems, e.g. CaO-MgO-Al2O3-SiO2 glass and found that Cr2O3
was a very effective additive for controlling the rate of nucleation
[18e20]. However, the effect of Cr2O3 on LAS glass-ceramic has not
been reported until now. Accordingly, the motive of this paper is to
explore the influence and mechanism of Cr2O3 addition on the
crystallization and properties of LAS system, and further improve
the mechanical property.
2. Experimental procedures
The chemical compositions of the Li2OeAl2O3eSiO2 glass-
ceramic were listed in Table 1, and reagent grade Li(OH)$H2O,
Al2O3, H3SiO3, ZnO, Mg(OH)2, Ca(OH)2, Cr2O3, Zr(OH)4, and H3BO3
with purity > 99% were chosen as the raw materials. LAS glass-
eceramic in this work was prepared by the melting method as
follow. After weighed accurately, the raw materials were ball milled
uniformly, and melted in a platinum crucible by an electric furnace
* Corresponding author. School of Microelectronics and Solid-State Electronics,
University of Electronic Science and Technology of China, Chengdu 610054, China.
E-mail address: boli@uestc.edu.cn (B. Li).
http://dx.doi.org/10.1016/j.jallcom.2016.09.043
0925-8388/© 2016 Elsevier B.V. All rights reserved.

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B. Li et al. / Journal of Alloys and Compounds 693 (2017) 9e15
Table 1
USA). The flexural strength was determined by an electromechan-
ical universal testing machine (MTS CMT6104, China). Microstruc-
tures of the cross-section for glass-ceramic were observed by a
scanning electron microscopy (SEM, FEI Inspect F50, USA). X-ray
diffractometer (XRD, PANalytical PW3040/60, Netherlands) using
Compositions of Li2OeAl2O3eSiO2 glass-ceramic (wt%).
Li2O
Al2O3
SiO2
ZnO þ MgO þ CaO
B2O3
ZrO2
Cr2O3
C1
C2
C3
C4
4
4
4
4
15
15
15
15
58
58
58
58
14
14
14
14
5
5
5
5
4
4
4
4
1
2
3
4
Cu-Ka radiation in the 2q
range of 20e80ꢀ in steps of 0.02ꢀ was
employed to detect the crystal phases.
3. Results and discussions
at 1500 ꢀC for 2 h. The molten glass was immediately quenched into
distilled water to form cullet. The cullet was grinded using alumina
ball and deionized water in the alumina jar for 0.5 h. After dried at
100 ꢀC, the obtained powders added different amount of Cr2O3
were mixed by ball milling with deionized water for 7 h. Then, the
dry powders prepared above were granulated after mixing with
5 wt% acrylic emulsion, and then uniaxially pressed under a pres-
Fig. 1 show the DSC curves of the four samples obtained at the
heating rates of 5, 10, 15 and 20 ꢀC/min. As we know, during the
sintering process of Li2OeAl2O3eSiO2, the phase transformation
occurs and experiences the following three steps: glass phase /
quartz solid solution / -spodumene solid solution / liquid
phase. The metastable -quartz solid solution must develop below
750 ꢀC, and otherwise it will change to
-spodumene solid solution.
Additionally, the phase diagram of SiO2 says the stable -quartz can
form in the temperature range of 230e570 ꢀC, or else it will
immediately transform to -quartz. Therefore, it was considered
that an exothermic peak at about 330 ꢀC may be caused by the
formation of -quartz solid solution. The glass transition temper-
ature (Tg) is approximately at 530 ꢀC on the DSC curve. Owing to the
crystallization of major phase -spodumene, significant
exothermic peak is present at around 750e800 ꢀC.
b-
b
b
sure of 25 MPa to obtain the green disks (
F
25 mm ꢁ 1 mm) and
b
bars (50 mm ꢁ 4 mm ꢁ 3 mm). Finally, these compacts were burn
out the binder at 450 ꢀC for 3 h, and then sintered in air at 800 ꢀC for
0.5 h.
b
a
To analyze the thermal reaction procedure of glass powders and
to calculate the crystallization kinetic parameters, the differential
scanning calorimeter (DSC, Netzsch STA 449C, Germany) was per-
formed at the heating rates of 5, 10, 15, and 20 ꢀC/min from 20 to
1100 ꢀC in air. The thermal expansion behavior and the coefficient
of thermal expansion (CTE) were measured using a dilatometer
(Netzsch DIL 402 PC, Germany) at a heating rate of 5 ꢀC/min from 20
to 600 ꢀC. The bulk density of the sintered samples was examined
by a precision density balance (And GF-300D, Japan) based on
Archimedes method. The dielectric constant and dielectric loss
were tested at 1 MHz using a precision LCR meter (Agilent 4284A,
b
b
a
The crystallization peak temperatures (Tp) at different heating
rates are marked in Fig. 1 and all the values are listed in Table 2. It
can be seen that the values of Tp shift to higher temperatures with
increasing the heating rate. Further, the activation energy for
crystallization of the glass samples could be decided by using the
Kissinger method [21], which is expressed as
Fig. 1. DSC curves at different heating rates for the glass samples (a) C1, (b) C2, (c) C3, (d) C4.

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B. Li et al. / Journal of Alloys and Compounds 693 (2017) 9e15
11
Table 2
Table 3
Tp values from DSC curves of LAS glass at different heating rates (ꢀC).
E and n values of LAS samples.
Heating rates (ꢀC/min)
C1
C2
C3
C4
Crystallization parameter
C1
C2
C3
C4
5
755.4
775.0
786.5
796.1
754.0
774.5
786.5
796.1
756.5
777.5
789.3
799.9
756.0
776.5
787.7
797.1
E (kJ/mol)
n
158.5
4.46
152.1
4.69
149.3
5.09
156.9
4.50
10
15
20
Fig. 3 is Ozawa plots ln[-ln(1-x)] versus lnb for the glass samples
related to exothermic peak shown in Fig. 1. The calculations of
crystallization index n are listed in Table 3. The values of n deter-
mined from the slopes of these plots vary between 4.46 and 5.09. It
is well known that the crystallization index is related to crystalli-
zation manner: n ¼ 1, surface crystallization, n ¼ 2, two dimension
crystallization, n ꢄ 3, volumetric crystallization. Thus, the crystal-
lization index n is more than 3 in this work, indicating that the
crystallization manner of samples is the volumetric crystallization.
XRD patterns of LAS with various Cr2O3 content sintered at
800 ꢀC are illustrated in Fig. 4. Obviously, Li2OAl2O37$5SiO2 (JCPDS
No. 53-1278) is the major phase. The secondary phase is CaMgSi2O6
(JCPDS No. 83-1817) and the third is ZrO2 (JCPDS No. 79-1768),
which was directly proved by the following calculation. When
Cr2O3 content ꢄ2 wt%, some diffraction lines belonging to the new
crystal phase Cr2O3 (JCPDS No. 82-1484) were found in samples C2-
C4. Further, the phase content and the crystallinity of LAS glass-
ceramic were estimated by the whole pattern fitting (WPF)
method using MDI Jade 6.5. As listed in Table 4, all samples doped
with Cr2O3 possessed higher crystallinity than 80%, which was
elevated from 81.43% to 86.80% by increasing Cr2O3 content. This
suggested that Cr2O3 addition could promote the crystallization of
LAS glass. Moreover, the Cr2O3 addition could decrease the content
of major phase Li2OAl2O37$5SiO2, but increase that of secondary
phase CaMgSi2O6. The third phase ZrO2 scarcely changed its con-
tent with varying the Cr2O3 addition. Besides, the result shows that
most chromium participated in the formation of the minor crystal
phase Cr2O3.
ꢀ
. ꢁ
E
RTp
ln Tp2
b
¼
þ C
(1)
where
b is the heating rate, E is the activation energy for crystal-
lization, R is the gas constant, and C is constant. Values of E can be
calculated using Eq. (1) by plots of ln(T2p/
b
) versus 1/Tp which are
shown in Fig. 2, and the results are given in Table 3. As we know, the
activation energy E is corresponding to the energy barrier of tran-
sition from glass to crystal. With more Cr2O3 addition, the activa-
tion energy E decreases from 158.5 to 149.3 kJ/mol. Specimen C3
has the lowest E value of 149.3 kJ/mol, which means that the glass
crystallization become easiest with 3 wt% Cr2O3 addition. It is
suggested that a proper amount of Cr2O3 is beneficial to the crys-
tallization, which is also consistent with the calculation of
crystallinity.
The value of crystallization index n can be determined from the
Ozawa equation [22].
d½ ꢂ lnð1 ꢂ xÞꢃ
¼ ꢂn
(2)
d ln
b
where x is the volume fraction crystallized at a fixed temperature T
for the heating rate of . In other words, x is the ratio of the partial
area at a given temperature T to the total area under the crystalli-
zation exotherm, and its value can be estimated from the exo-
therms in Fig. 1. The plot of ln [-ln(1-x)] versus ln
straight line whose slope is n.
b
b should be a
Fig. 5 shows SEM micrographs for the fractured LAS glass-
Fig. 2. Relationship between ln(T2p/
b) and 1/Tp for the glass samples (a) C1, (b) C2, (c) C3, (d) C4.

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B. Li et al. / Journal of Alloys and Compounds 693 (2017) 9e15
Fig. 3. Relationship between ln[-ln(1-x)] and ln
b
for the glass samples (a) C1, (b) C2, (c) C3, (d) C4.
ceramic samples with different amount of Cr2O3 sintered at 800 ꢀC.
The dense microstructure was formed for all samples doped with
Cr2O3 and only a few pores were existed inside. The number of
pores was gradually reduced with the increase of Cr2O3 content.
Thus, the sufficient Cr2O3 addition could further improve the
densification of microstructure for LAS glass-ceramic. Back scat-
tered electron mode was adopted to take SEM images, which allows
clear recognition of the formed crystals and the residual glass via
different colors. It was observed that the dark gray regions were
surrounded by the light gray regions. In order to clarify the
constitution of these two regions, EDS analysis was conducted at
the corresponding two points on Fig. 5c for sample C3, and the
spectra were presented in Fig. 6a and b, respectively. Lithium and
Boron, as light elements, cannot be detected by EDS. Both two
patterns contained the same elements of Si, Al, Zn, Mg, and Zr.
However, the element Cr was not detected in these points, which
may result from mainly precipitating as the minor crystal phase
Cr2O3. It is important to point out that the atomic percentage of Al
and Si for point A, in Fig. 6a, is nearly equaled to the major crystal
phase Li2OAl2O37$5SiO2. As we known, glass-ceramic is usually
composed of the crystal phase as well as the residual glass phase.
This demonstrated that the dark gray zone was the major crystal
phase Li2OAl2O37$5SiO2 and the light gray zone was the residual
glass phase. The small amount of glass phase bonded the crystals
together, guaranteeing the mechanical strength. It was clearly seen
that the crystals almost exhibited irregular shape and non-uniform
size. The crystal size varied in the range of 1e2
mm and the large
one reached about 4 m. The homogeneous dispersion of crystals
m
appeared in the interior of glass-ceramics, indicating that the
crystallization mechanism is bulk crystallization. This result is in
agreement with the above calculation of crystallization index n.
Besides, the widely distributed crystals covered the 80% area of SEM
picture, which also verified the crystallinity calculated from XRD
analysis.
Bulk density, shrinkage rate, and mechanical properties of the
LAS composites sintered at 800 ꢀC as a function of Cr2O3 content are
illustrated in Fig. 7. All sintered samples at 800 ꢀC could obtain the
higher bulk density than 2.5 g/cm3 and form the dense micro-
structure (in Fig. 5), and the change in density is not significant as a
Table 4
Phase content and crystallinity calculated from XRD.
Sample
Phase content (wt%)
Crystallinity (%)
Li2OAl2O37$5SiO2
CaMgSi2O6
ZrO2
Cr2O3
C1
C2
C3
C4
85.5
82.1
80.8
79.5
11.7
13.6
14.1
14.4
2.8
2.9
2.8
3.0
0
81.43
83.50
86.80
84.68
1.4
2.3
3.1
Fig. 4. XRD patterns of LAS glass-ceramics with different Cr2O3 content sintered at
800 ꢀC.