Effects of the ZBS addition on the sintering behavior and microwave dielectric properties of 0.95Zn2SiO4-0.05CaTiO3ceramics

Article abstract of DOI:10.1016/j.jallcom.2016.11.420

The effects of zinc borosilicate addition (denoted as ZBS) on densification and dielectric properties of a binary composite of Zn₂SiO₄-CaTiO₃(denoted as ZSCT) ceramics have been investigated as a function of ZBS content and sintering temperature. Densities of the specimens were enhanced with an increase of ZBS and then decreased. High-density ZSCT?+?20.0?wt%ZBS sintered at 925?°C showed promising microwave dielectric properties: relative dielectric constant (?_r)?=?7, quality factor (Q?×?f)?=?19,265?GHz?at 15.5?GHz, and temperature coefficient of resonant frequency (τ_f)?=??21?ppm/°C, which demonstrated a potential candidate for application of low temperature co-fired ceramics field in millimeters and THz range.

Full text of DOI:10.1016/j.jallcom.2016.11.420

Accepted Manuscript
Effects of the ZBS addition on the sintering behavior and microwave dielectric
properties of 0.95Zn SiO -0.05CaTiO ceramics
2
4
3
Zhangzhao Weng, Rengang Guan, Zhaoxian Xiong
PII:
S0925-8388(16)33903-2
10.1016/j.jallcom.2016.11.420
JALCOM 39917
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 19 October 2016
Revised Date: 27 November 2016
Accepted Date: 30 November 2016
Please cite this article as: Z. Weng, R. Guan, Z. Xiong, Effects of the ZBS addition on the sintering
behavior and microwave dielectric properties of 0.95Zn SiO -0.05CaTiO ceramics, Journal of Alloys
2
4
3
and Compounds (2017), doi: 10.1016/j.jallcom.2016.11.420.
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ACCEPTED MANUSCRIPT
Effects of the ZBS addition on the sintering behavior and
microwave dielectric properties of 0.95Zn SiO -0.05CaTiO
2
4
3
ceramics
Zhangzhao Weng*, Rengang Guan, Zhaoxian Xiong.
Municipal Key Laboratory of Electronic Ceramics and Components, College of Materials,
Xiamen University, Xiamen 361005, China
Abstract:
The effects of zinc borosilicate addition (denoted as ZBS) on densification and
dielectric properties of a binary composite of Zn SiO -CaTiO (denoted as ZSCT)
2
4
3
ceramics have been investigated as a function of ZBS content and sintering
temperature. Densities of the specimens were enhanced with an increase of ZBS and
then decreased. High-density ZSCT+20.0wt%ZBS sintered at 925°C showed
promising microwave dielectric properties: relative dielectric constant (Ɛ )=7, quality
r
factor (Q×f)= 19,265GHz at 15.5 GHz, and temperature coefficient of resonant
frequency (τf)=-21 ppm/°C, which demonstrated a potential candidate for application
of low temperature co-fired ceramics field in millimeters and THz range.
1
. Introduction
The ongoing development of modern communication technology, such as high
density, high RF and fast digital applications requiring hermetical packaging and good
thermal management, has pushed the continuing miniaturization of microwave
devices. Recently, low temperature cofired ceramics (LTCC) multilayer devices have

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been intensively investigated to reduce device size by enabling a significant number
of circuit components to be integrated within a module in a multilayer
substrate[1][2][3]. In LTCC, a low sintering temperature lower than the melting point
of metal electrodes (961°C for Ag) is critical instead of using more expensive
electrodes such as Ag-Pd binary or Pt-Pd-Au ternary alloys. Unfortunately, most of
well-known microwave ceramics with good dielectric properties in the microwave
range need high sintering temperatures, which put it a constraint on LTCC
application.
On the other hand, the utilizable frequency region has been expanded to
millimeterwaves and even extends to THz band for the shortage of conventional
frequency regions. Therefore, in the millimeterwave region or THz band, the
permittivity (Ɛ ) value is expected to be small for no need for miniaturization and
r
reducing time delay[4]. However, the lack of advanced materials in the
millimeterwaves and THz band is a major constraining factor. Among the low Ɛr
materials, Zn SiO ceramics are particularly promising candidates[5] because of high
2
4
quality factor (Q×f) value and adjustable temperature coefficient of resonant
frequency. But they were produced by the cold isostatic press (CIP) method, which
cannot be used for mass production and the high sintering temperature (~1,300°C) is
an another limitation for its application in LTCC technology[6][7][8].
In this study, the microwave dielectric ceramics based on Zn SiO was chosen as
2
4
the host material. To compensate the τf value (-61 ppm/°C) of ZS ceramics[5], CaTiO3
with positive τf value (+859 ppm/°C)[9][10] was added to form a composition of

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0
.95Zn SiO +0.05CaTiO ceramics [11] with near-zero τf value and the Ɛ values are
2
4
3
r
below 7.4, which are lower than that of other Zn SiO systems with near-zero τf
2
4
value[12]. Zinc borosilicate glass used as sintering agent are the most commonly used
glass materials in glass/ceramic composites for microelectronic packaging[13]. In this
work, the densification and microwave dielectrics of ZSCT with different amount of
ZBS addition were investigated. The intention of this study is to systematically
investigate the dielectric properties of ZBS doped ZSCT ceramics prepared by
solid-state reaction method and consider the possibility of the ceramics in modern
electronic systems for LTCC application. Since the results of the study greatly depend
on the quality of the ceramics, emphasis has been placed on the relationship between
microwave dielectrics properties of ZSCT ceramics with different ZBS and the
sintering temperature.
2
. Experimental procedure
The ZBS addition with the composition of 60.0%ZnO, 30.0%B O , and 10.0%SiO
2
2
3
(
in mol) was directly mixed with high-purity oxides, ZnO (99%), SiO (99%),
2
B O (99%) from the West Long Chemical Co., Ltd., Guangdong, China. Zn SiO and
2
3
2
4
CaTiO compounds were individually synthesized by a conventional solid-state
3
reaction method through using analytical-grade oxide powders(>99.99%) of ZnO,
SiO , TiO ,CaCO as starting materials. They were mixed according to the nominal
2
2
3
composition Zn SiO (denoted as ZS), CaTiO (denoted as CT), respectively, and
2
4
3
ball-milled in a polyethylene bottle with agate balls using distilled water as a medium.
The mixture was then dried and calcined at 1150� and 1100� for 2h respectively.

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The calcined powders were re-milled for 4h according to the desired composition:
0
.95Zn SiO -0.05CaTiO doped with different contents (5wt.%, 10.0wt%, 15wt.%
2 4 3
and 20wt.%) of ZBS addition. The powder mixture, with poly vinyl alcohol (PVA) as
an organic binder(0.70wt.%) and granulated was subsequently uniaxially pressed into
cylindrical compacts 13mm in diameter and 5.2-6.8mm in thickness using a uniaxial
press (200 MPa). The green pellets were heated at 1.5°C/min up to 600°C and held for
1
h to remove the binder. The samples were sintered at 855-1065°C for 2h with a
heating rate of 5°C /min in air after de-binding, and then cooled to room temperature.
The bulk density of the sintered samples was measured by the Archimedes' method
using distilled water as the immersion medium. The crystalline phase and
microstructure were determined by X-ray powder diffraction(XRPD). Two-theta scans
between 10° and 80° were determined by a Bruker-AxsD8 diffractometer using
-1
Cu-Kα radiation at a scan rate of 1° min , operated at 40 kV and 40 mA. The
microstructure observation and an energy dispersive spectra (EDS) were performed
from the fractured surface by scanning electron microscopy (SEM; Hitachi SU70,
Japan).
The dielectric constants and unloaded Q values were measured in the TE011 mode
using the Hakki and Coleman method[14], where a cylindrically shaped specimen is
positioned between two silver plates. An E8362B network analyzer was used as the
measuring system. The temperature coefficient of frequency (τf ) values were
evaluated in the temperature range between 20 and 80°C, and were calculated by the
following equations:

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ꢀ80
ꢁꢀ20
6
τf
=
× 10 (ppm/°C)ꢂꢂꢂꢂꢂꢂꢂꢂꢂꢂꢂ(1);
×60
ꢀ20
Where f represent the resonant frequency. Subscripts of 20 and 80 are referred to the
testing temperature, respectively.
3
. Results and discussion
Fig.1 shows the XRD patterns of the ZSCT+xwt.%ZBS ceramics with 5.0≤x≤ 20.0,
sintered at various temperatures for 2h. It is clearly observed that the peaks of all the
samples annealed at various temperatures are good match with the standard ICSD
card for Zn SiO (no.37-1485, willemite) and CaTiO (no.82-0228, Perovskite) , as
2
4
3
anticipated. The peaks indicated the presence of Zn SiO as the main crystalline phase,
2
4
and CaTiO as the minor crystalline phase.
3
Fig.1 XRD patterns of 0.95Zn SiO -0.05CaTiO system with different contents of ZBS sintered at
2
4
3
various temperatures: (a) 5.0wt%ZBS, sintered at 1030°C, (b) 10.0wt%ZBS, sintered at 995°C, (c)
5.0wt%ZBS, sintered at 960°C, (d) 20.0wt%ZBS, sintered at 925°C for 2h.
1

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The relative density of ZSCT ceramics with various amount of ZBS addition at
different sintering temperature for 2 h are shown in Fig.2. It appears that the density
of samples with different content of addition (5.0≤x≤20.0) increased firstly,
subsequently decreased with further increasing of sintering temperature. The results
also demonstrate that the onset of densification mechanism for the specimens moves
towards much lower temperatures with the addition of glasses. The densification
temperature of ZSCT+xwt%ZBS (x=15,20) was around 960°C, while that of
ZSCT+10.0wt%ZBS and ZSCT+5.0wt%ZBS were around 995°C and 1030°C
respectively. This corresponded with the fact that the lowest pore density and the
uniform grain size sintered at the densification temperature shown in Fig.3.
Cross-sectional SEM images with backscattering mode in Fig.3 demonstrate that
these specimens sintered at the densification temperature have dense microstructures.
To further identify the minor crystalline phase (CaTiO ) from the main crystalline
3
phase(Zn SiO ), EDX analysis was performed and the testing results of the
2
4
ZSCT+5.0wt%ZBS ceramic sintered at 1030 °C for 2h are shown in table 1.
According to the semi-quantitative molar ratio of Zn/Si, the large rounded-shape
grains (1) were identified as Zn SiO due to a Zn/Si ratio of about 2:1. The small
2
4
square block-shape gains (2) were identified as CaTiO due to a Ca/Ti ratio of about
3
1
:1.
The densification mechanism of the composites can be explained as liquid-phase
sintering by considering ZBS addition is uniformly dispersed in the glass phase matrix
for the well wettability between the glass and ceramic powder[15][16], which could

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act as lubrication during sintering process, wetting solid particles, and providing
capillary pressure between them, thus resulting in a faster grain growth of
ceramics[17]. However, the overquik grain growth would preclude the pore venting
process as further increase sintering temperature and eventually decrease the relative
density in ceramics.
The theoretical density (TD) of ZSCT was calculated by the following equation:
ν
1ꢃ1
1ꢃ1ꢄ
ν
ν
2ꢃ2
2ꢃ2
ρ
=ꢂν
(2);
ꢅ
ꢄ
ꢅ
2
ρ
1
ρ
Where
ρ, M and ν represent the theoretical density, mole mass and mole ratio,
respectively, of each part of mixture. Subscripts of 1 and 2 are referred to the ZS and
3
CT, respectively. Here, ν1=0.95, ν2=0.05; M =222.87g/mol, ρ1=4.252g/cm (estimated
1
3
from JCPDS no. 37-1485); M =135.98 g/mol, ρ2=4.030 g/cm (estimated from
2
3
JCPDS no. 82-0228). The TD value of 0.95Zn SiO -0.05CaTiO was 4.2447 g/cm
2
4
3
calculated by equation (2). For ZSCT+20.0wt%ZBS ceramics, the 94.5%, 96.2%,
9
9
6.9%, 97.2%, and 96.8% of TD were obtained with ease at 855, 890, 925, 960 and
95°C, respectively.

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98
96
94
92
90
88
86
5
.0wt% ZBS
10.0wt% ZBS
15.0wt% ZBS
20.0wt% ZBS
855
890
925
960
995
1030
1065
Sintering Temperature ( C)
°
Fig.2 Relative density of the ZSCT+xwt.%ZBS ceramics as a function of sintering temperature
and amount of ZBS addition
3
µm
3 µm
3
µm
3 µm
Fig.3 Cross-sectional scanning electron microscopy images of ZSCT+xwt.%ZBS ceramics with
backscattering mode as a function of sintering temperature and amount of ZBS addition: (a)x=5,
1030°C /2h, (b)x=10, 995°C /2h, (c) x=15, 960°C /2h, (d)x=20, 925°C /2h.

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Table 1
EDX analysis of ZSCT+5.0wt%ZBS ceramic sintered at 1030 °C for 2h.
Grain 1
Zn/Si
Grain 2
Ca/Ti
The two main elements A/B
Molar ratio of A/B
Composition
1.96
0.94
Zn SiO
CaTiO3
2
4
Fig.4 shows the Ɛ , Q×f, and τf values of the ZBS-doped ZSCT ceramics sintered at
r
various temperatures for 2h. The Ɛ and Q×f values of all specimens with different
r
ZBS contents increased firstly, subsequently decreased with the increasing of
sintering temperature, which displayed the same trend of relative density with the
sintering temperature. However, the peak values of Ɛ and Q×f appeared at a lower
r
sintering temperature than that of relative density appeared which probably account
for the formation of a large amount of liquid phase as sintering temperature above
9
00°C. Moreover, the maximum Ɛ value of the ZBS-doped ZSCT ceramics decreased
r
as the amount of ZBS increased for the higher ratio of glass with the relative low Ɛ to
r
ZSCT ceramics. According to Fig.4(a), the Q×f values of the sample doped with
2
0.wt%ZBS reached the maximum (19,265GHz at 15.5 GHz) when the sintering
temperature was 925 °C and decreased monotonically with the increasing of sintering
temperature. Similar trend was obtained for the specimens with 15.wt%ZBS and the
maximum Q×f value (13,013GHz at 15.0 GHz) was obtained as the sintering
temperature was 960 °C. Additionally, the Q×f value (25,674GHz at 14.8 GHz) of the
ZSCT+5.0wt%ZBS ceramics were much larger than that of other samples, although
the relatively high sintering temperature is beyond the range of LTCC. Generally,
many factors are commonly believed to decide the microwave dielectric properties

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and can be divided into two parts: the intrinsic loss and the extrinsic loss. Intrinsic
loss is mainly caused by lattice vibration modes while extrinsic loss is dominated by
lattice defect, crystallizability, cation ordering second phases, impurities, oxygen
vacancies, inner stress, and densification or porosity. In ZSCT-ZBS system, it is
difficult to determine the key influential factor, however, it's not doubt that the
melting mechanism and the last exist micro-morphology of ZBS play crucial role in
this system, especially for the ZSCT ceramic with large amount of ZBS addition.
The τf values of ZSCT ceramics with different ZBS contents sintered at various
temperatures are depicted in Fig.4(b). It's obvious demonstrated that the τf value
decreased apparently with the amount of ZBS addition increased. The variation of the
τf values with different ZBS contents range between -2 and -21 ppm/°C and the τf
value of specimen with 20.0wt% ZBS content sintered at 925°C is -21 ppm/°C.
7
.4
a)
7
.2
7.0
6.8
6.6
2
5000
0000
2
1
5000
0000
1
5.0wt% ZBS
1
1
2
0.0wt% ZBS
5.0wt% ZBS 5000
0.0wt% ZBS
855
890
925
960
995
1030
1065
Sintering Temperature (°C)

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0
4
8
b)
-
-
5.0wt% ZBS
1
1
2
0.0wt% ZBS
5.0wt% ZBS
0.0wt% ZBS
-12
-16
-20
855
890
925
960
995
1030
1065
Sintering Temperature ( C)
°
Fig.4 Variation of microwave dielectric properties of the ZSCT+xwt.%ZBS ceramics as a function
of sintering temperature: (a) relative dielectric constant (Ɛ ) and Quality factor (Q×f), (b)
r
temperature coefficient of the resonant frequency (τf).
In order to investigate the reactivity between ZSCT and ZBS, green compacts of ZBS
was placed on the top of porous ZSCT green pellets and cofired at 925°C for 2h in air.
Fig.5 shows the samples before and after firing. ZBS showed nearly nonreactivity
with ZSCT and a negligible reaction product could be found by interface SEM images
in Fig.6. The phenomena confirm that the ZBS addition could be an excellent
candidate for ZSCT LTCC. Furthermore, it is clearly observed that sintered ZBS
possessed porous structure, which put constraints on the further improving microwave
properties of ZSCT with a large amount of addition.
a)
b)
ZBS
ZBS
ZSCT
ZSCT
Fig.5 Compact of ZBS and ZSCT (a) before firing, (b) after firing at 925°C for 2h.

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3
00 µm
Fig.6 SEM micrographs of interface between ZSCT and ZBS after firing at 940°C for 2h.
4
. Summary
The effects of zinc borosilicate addition (ZBS) on densification and dielectric
properties of a binary composite of Zn SiO -CaTiO (ZSCT) ceramics have been
2
4
3
investigated as a function of ZBS content and sintering temperature. The results
indicated that the onset of densification mechanism for the specimens moves towards
much lower temperatures with the addition of glasses and the densities of the
specimens were enhanced with an increase of ZBS and subsequently decreased. In
addition, ZBS showed nearly nonreactivity with ZSCT and a negligible reaction
product could be found. Moreover, high-density ZSCT+20.0wt%ZBS sintered at
9
25°C showed promising microwave dielectric properties: relative dielectric constant
Ɛr)=7, quality factor (Q×f)= 19,265GHz at 15.5 GHz, and temperature coefficient of
resonant frequency ( f)=-21 ppm/°C, which demonstrated a potential candidate for
(
τ

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application of low temperature co-fired ceramics field in millimeters and THz range.
Acknowledgements
This work was financially supported by a grant (Grant No. 0044-K5100001) from Xiamen
Municipal Key Laboratory of Electronic Ceramics and Components.
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1
9
. High-density ZSCT ceramic was obtained with the sintering temperature below
50°C.
2
. ZSCT ceramics with addition of ZBS show the well microwave properties in
LTCC.
3
. ZBS shows nearly nonreactivity with ZSCT and a negligible reaction product could
be found.