Newly developed polytetrafluoroethylene composites based on F8261-modified Li2Mg2.88Ca0.12TiO6 powder

Article abstract of DOI:10.1016/j.jallcom.2019.06.109

Li₂Mg_2.88Ca_0.12TiO₆ (LMCT) was modified by silane coupling agent (C₁₄H₁₉F₁₃O₃Si, F8261), and then filled in polytetrafluoroethylene suspension (PTFE) using hot treating approach. The weight fraction of LMCT varied from 30 to 70 wt %. XRD patterns presented the crystal phase of LMCT powders. The analyses of TEM, XPS, and FTIR were performed to characterize the surface states of LMCT fillers. The effects of LMCT filling content on the microstructure and dielectric properties were also investigated. Several theoretical models were discussed and compared with the dielectric constant of LMCT/PTFE substrates with respect to different LMCT content. Furthermore, the temperature and frequency dependence of dielectric constant were also considered on the composites. At last, the LMCT/PTFE composite substrates possessed relatively high dielectric constant (ε_r = 4.33) and extremely low dielectric loss (tan δ = 0.001) at filler loading of 50 wt % LMCT.

Full text of DOI:10.1016/j.jallcom.2019.06.109

Journal of Alloys and Compounds 803 (2019) 145e152
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Newly developed polytetrafluoroethylene composites based on
F8261-modified Li Mg2.88Ca0.12TiO powder
2
6
Fuchuan Luo b, Shuren Zhang a b, Bin Tang a
a
,
,
,
b
,
*
, Ying Yuan a
,
b
,
**
, Zixuan Fang a
, b
a
National Engineering Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Jianshe Road,
Chengdu, 610054, PR China
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Jianshe Road, Chengdu,
b
610054, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 February 2019
Received in revised form
Li Mg2.88Ca0.12TiO6 (LMCT) was modified by silane coupling agent (C14H19F13O Si, F8261), and then filled
2
3
in polytetrafluoroethylene suspension (PTFE) using hot treating approach. The weight fraction of LMCT
varied from 30 to 70 wt %. XRD patterns presented the crystal phase of LMCT powders. The analyses of
TEM, XPS, and FTIR were performed to characterize the surface states of LMCT fillers. The effects of LMCT
filling content on the microstructure and dielectric properties were also investigated. Several theoretical
models were discussed and compared with the dielectric constant of LMCT/PTFE substrates with respect
to different LMCT content. Furthermore, the temperature and frequency dependence of dielectric con-
stant were also considered on the composites. At last, the LMCT/PTFE composite substrates possessed
1
June 2019
Accepted 9 June 2019
Available online 11 June 2019
Keywords:
PTFE
LMCT
Surface characterization
Dielectric properties
relatively high dielectric constant (ε
loading of 50 wt % LMCT.
r
¼ 4.33) and extremely low dielectric loss (tan
d
¼ 0.001) at filler
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
over a wide range of frequencies [8]. However, the main disad-
vantage of PTFE is the negative temperature coefficient of dielectric
ꢀ
Currently, the development and evolution of high integrated
microwave circuit devices are becoming gradually confined by
printed wiring board (PWB) substrates which are mainly comprised
of organic polymer and inorganic fillers. The PWB substrates,
especially the polymer based and ceramic filled substrates, have
played a vital role in circuit design, which requires high working
frequency, good temperature stability, easy processing and excel-
lent dielectric properties [1]. To achieve substrate materials with a
wide range of properties, various kinds of polymers and ceramic
fillers with different dielectric property have been explored [2e7].
Among these polymers, polytetrafluoroethylene (PTFE) is a ther-
moplastic polymer with excellent electric properties of low
constant (
t
ε
~-400 ppm/ C) making it difficult to be used in micro-
wave circuit devices. Generally, ceramic fillers have suitable
dielectric constant, low loss tangent and relatively high thermal
conductivity [9e11]. According to our previous reports [12e15],
Li
suitable dielectric constant (ε
(Q ꢁ f ¼ 100000 GHz), and extremely low dielectric loss
(tan
2
Mg2.88Ca0.12TiO
6
(LMCT) is a promising microwave ceramic with
r
¼ 17.9), splendid quality factor
d
¼ 0.0001). It is likely that LMCT filled PTFE composites will
also exhibit excellent dielectric properties in the application of
microwave devices. During the past decade, there were many
studies on tailoring the dielectric, mechanical and thermal prop-
erties of polymers based and ceramics filled composites [16e18].
There is a strong demand to acquire an optimal composite which
could simultaneously fulfill diverse applications.
r
dielectric loss (tan d~0.0003) and stable dielectric constant (ε ~2.1)
However, due to the incompatible surface microstructure, it is
difficult for organic polymer to cover the surface of inorganic fillers
without any defects. Bad adhesion between polymer matrix and
fillers adversely affects the comprehensive properties of the com-
posites. Traditional PWB substrates have limited dielectric and
mechanical properties in the most of microwave devices, which is
mainly due to the difference surface structure existed between
*
Corresponding author. State Key Laboratory of Electronic Thin Films and Inte-
grated Devices, University of Electronic Science and Technology of China, Jianshe
Road, Chengdu, 610054, PR China.
** Corresponding author. State Key Laboratory of Electronic Thin Films and Inte-
grated Devices, University of Electronic Science and Technology of China, Jianshe
Road, Chengdu, 610054, PR China.
E-mail addresses: tangbin@uestc.edu.cn (B. Tang), yingyuan@uestc.edu.cn
(
Y. Yuan).
https://doi.org/10.1016/j.jallcom.2019.06.109
925-8388/© 2019 Elsevier B.V. All rights reserved.
0

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F. Luo et al. / Journal of Alloys and Compounds 803 (2019) 145e152
organic polymer and inorganic filler [19]. The influences of various
coupling agents have been studied to improve the interface of
ceramic filled polymer composites [19e23]. The coupling agent
changes the surface characterizations of inorganic fillers by grafting
molecular chain on the surface of filler particles. Hence, coupling
agent acts as a “bridge” between inorganic fillers and organic
polymers. Many studies have been conducted to research the in-
fluences of coupling agents on the composites. Majority of these
studies focused on the final performances of the composites.
However, to date, few investigations have been carried out to
discuss the surface states and characterizations of the coupling
agent modified fillers.
F8261 were obtained. The schematic processes of surface modifi-
cation on LMCT powders using F8261were presented in Fig. 1.
The PTFE suspension and baked LMCT were weighed precisely
according to the weight fraction of LMCT/PTFE composites, blended
uniformly using high speed stirring for 0.5 h and then baked at
ꢀ
120 C for 24 h to eliminate organic solvent and water. Thereafter,
the baked blend was grinded using high speed grinder. The grinded
powder was axially pushed into a square slot model under the
pressure of 20 MPa for 30 s. At last, the obtained square slices were
ꢀ
fold and baked at 380 C for 2 h with impressed pressure of 20 MPa.
The crystal phase structure of LMCT particles and LMCT filled
substrate composites were recorded by X-ray diffraction (XRD)
In present work, LMCT particles were used as fillers to adjust the
properties of PTFE based composite substrates. LMCT powders
were fabricated using typical solid state ceramic approach. The
weight fraction of LMCT changed from 30 wt % to 70 wt %.
using CuKa radiation (Philips x'pert Pro MPD, Netherlands). Scan-
ning electron microscopy (SEM, model JEOL JSM-6490) was per-
formed to observe the morphology of cross section of the
composite substrates. X-ray photoelectron (XPS, Al, Ka, Thermo
C
14
H
19
F
13
O
3
Si (F8261) was utilized as coupling agent to optimize
Scientific) was used to investigate the surface characterizations of
untreated LMCT and modified LMCT powders. Survey scan spectra
of untreated and modified LMCT powders were measured range
from 0 eV to 1350 eV. In addition, a high resolution scanning on
carbon element was operated to observe the carbon character
states of LMCT powders. Fourier transform infrared (FTIR, Nicolet
5700) spectra of untreated LMCT together with modified LMCT
were also presented to study the chemical bonds on the surface of
ceramics. Transmission electron microscopy (TEM, FEI Tecnai G2
F30 S-TWIN, America) was operated to present the morphology of
both untreated and modified LMCT powder as well as the interface
structure between LMCTand PTFE. The general dielectric properties
of PTFE based composites were measured using stripline resonator
approach according to IPC-TM-650 2.5.5.5 [24].
the surface of LMCT filler. The surface characterization of untreated
LMCT and modified LMCT was investigated in detail in this paper.
The dielectric properties and microstructure of the composite
substrates were also discussed at length.
2
. Experiment
2 6
2.1. Fabrication of Li Mg2.88Ca0.12TiO powders
To fabricate Li
ders with at least 90% purity of CaCO
TiO and Li CO were selected as raw materials. All kinds of raw
materials were weighed on the basis of stoichiometric ratio. Then
mixed and grinded in alcohol solution for 5 h with ball milling. The
2
Mg2.88Ca0.12TiO
6
particles, reagent grade pow-
3
(MgCO Mg(OH) $5H O,
3
)
4
2
2
2
2
3
ꢀ
obtained mixtures were baked and calcined at 1275 C for 6 h. Later,
3. Results
the calcined powders were reground for 2 h to acquire LMCT
powders with particle size of 15
m
m. After the milling process, the
suspensions were baked at 70 C for 12 h. Finally, the LMCT parti-
cles with average size of 15 m were fabricated.
Fig. 2 shows the XRD pictures of LMCT powder and LMCT/PTFE
composites with different weight content of LMCT. XRD charac-
ꢀ
m
terizations revealed that Li
2
Mg
3
TiO
6
and CaTiO
3
(JCPDS #
8
9e6949) phases coexisted in LMCT. The diffraction peaks
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2.2. Preparation of LMCT/PTFE substrates
appearing at 2
q
¼ 37.14 , 43.15 , 62.67 , 75.16 and 79.15 were
assigned to (111), (200), (220), (311) and (222), respectively. These
To tailor the dielectric properties and mechanical strength of the
peaks should belong to the reflections of LMCT powders. Peaks at
ꢀ
ꢀ
composite substrates, the sintered LMCT powders were selected as
dopant, and mixed into PTFE (China, Dupont) suspension. The main
performances of PTFE and LMCT were listed in Table 1.
2
q
¼ 17 and 32 corresponded to PTFE colloform (#JCPDS number
00-054-1595). As shown in Fig. 2, the diffraction peak intensity of
PTFE (2
loading of LMCT. On the contrary, the characteristic peak of LMCT
located at 62.67 displayed an increasing trend with the filling of
ꢀ
q
¼ 17 ) experienced a continuous decrease with the further
In this work, F8261 (Japan, TCI Corporation) was chosen as
silane coupling agent to optimize the characteristics of ceramic
fillers, due to the similar molecule structure between F8261 and
PTFE. First, F8261 and un-treated LMCT were weighed on the basis
of our previous works. After weighing, F8261 was hydrolyzed in a
solvent consisted of alcohol and deionized water with the ambient
ꢀ
LMCT particles. In addition, the peak shape of LMCT phase in LMCT/
PTFE composites kept highly consistent with LMCT powder, indi-
cating the chemical stability of LMCT material.
Fig. 3 presents the survey scan XPS spectra of the untreated
LMCT and modified LMCT powders. Peaks at 531, 458, 350, 285 and
49 eV corresponded to O1s, Ti2p, Ca2p3, C1s and Li1s, respectively. The
spectrum of modified LMCT powder possessed peaks at 836 and
689 eV, which were not found in untreated LMCT spectrum.
Furthermore, these peaks mentioned above were related to the
bonding energies of FAuger and F1s, respectively. The observed sig-
nals of FAuger and F1s peaks in modified LMCT should be originated
from the F element in F8261. The finding indicated that LMCT
particles were “covered” by F element after the modification pro-
cess performed on LMCT using F8261, resulting in a strong hydro-
ꢀ
temperature of 55 C for 1 h. In this work, the content of F8261 was
fixed at 2 wt % of untreated LMCT powders. The volume fraction of
deionized water was calculated precisely to guarantee and promote
the hydrolysis of F8261. Next, the weighed LMCT powders were
poured and blended in hydrolyzed F8261 solution by high speed
stirring for 2 h. The homogeneous blend was further baked in
ꢀ
drying oven with 120 C for 3 h. Finally, LMCT powders treated with
Table 1
The performances of LMCT and PTFE.
phobicity. The presence of
F element characteristic peaks
Properties
PTFE
LMCT
confirmed that the functional groups of F8261 have grafted on the
surface of LMCT, successfully.
The analyses of XPS narrow scanning on carbon element
together with peak fitting on untreated and modified LMCT were
shown in Fig. 4. According to the binding energies of the peaks [4],
Dielectric constant
Dielectric loss
Density/(g/cm )
2.1
0.0003
2.2
34
0.0001
3.43
7
3
ꢂ6 ꢂ1
Thermal expansion coefficient/(10
K
)
109

F. Luo et al. / Journal of Alloys and Compounds 803 (2019) 145e152
147
Fig. 1. Schematics illustration of the interaction between LMCT and F8261.
Fig. 2. XRD patterns of LMCT powder and LMCT/PTFE composites with various weight
Fig. 3. XPS spectra of (a) untreated LMCT and (b) modified LMCT.
percentage of LMCT.
F8261 were successfully transferred to the surface of LMCT
particles.
the C1s spectra of untreated LMCT and modified LMCT were
deconvoluted into six types, and these types were represented by
Fig. 5 presents the FTIR patterns of untreated LMCT and modi-
2ꢂ
ꢂ1
various colored lines: (1) C-F
3
, (2) C-F
2
, (3) C-F, (4) CO
3
, (5) C1s, (6)
fied LMCT powders. The absorption peak at ca. 3650 cm
was
C-H (or C). The obtained C1s peak shape of untreated LMCT pow-
ders, as shown in Fig. 4(a), was mainly contributed by calibration
during the test process. Based on the peak fitting, the C1s peak
corresponded to the Ti-OH bond indicating that the surface of LMCT
was coated by hydroxyl (-OH). The Ti-OH peak disappeared in the
FTIR pattern of modified LMCT. According to Fig. 1, this was due to
2ꢂ
shape of untreated LMCT was comprised of C-C, C1s and CO
Obvious changes in C1s peak shape were detected in modified LMCT
Fig. 4(b)) when compared with untreated LMCT (Fig. 4(a)).
3
.
the reaction between F8261 and LMCT ceramics. The peak at ca.
ꢂ1
3640 cm
in both patterns were mainly contributed by the
(
ambient moisture during the measure process. Besides, peaks at
ꢂ1
ꢂ1
Meanwhile, the peak fitting analysis was also applied to the spec-
trum of modified LMCT. It can be concluded from Fig. 4(b) that the
1490 cm and 1440 cm referred to the asymmetric deformation
vibration and bending vibration of CO , respectively. The CO
2
2
main species of C element of modified LMCT were C-F
3
, C-F
2
, C-F,
probably came from the raw materials of Li
peaks at 1250 cm was the characterization absorption band (C-F)
of F8261, demonstrating that F8261 reacted and grafted on the
surface of LMCT powder. Furthermore, peaks at 1140 and 474 cm
were correlated with asymmetric stretching vibration and bending
2
CO
3
and CaCO
3
. The
2ꢂ
ꢂ1
CO
states of C-C, C1s and CO
process. The other states of carbon, such as C-F
were probably originated from -(CH )-(CF -CF
3
, C-H (or C) and C1s. Among these species, the observed carbon
2
ꢂ
3
were also contributed by the calibration
, C-F , C-F and C-H
chains grafted on
ꢂ1
3
2
2
2
)
5
3
ꢂ1
the surface of modified LMCT. These detected carbon bonds were
vibration of Si-O-Ti bond. The absorption band of 667 cm was
from F8261 coupling agent, indicating that the functional groups of
corresponding to the vibration of Ti-O bond. The existence of Si-O-

148
F. Luo et al. / Journal of Alloys and Compounds 803 (2019) 145e152
2
ꢂ
Fig. 4. Carbon peak fitting of the XPS narrow scan spectrum of (a) untreated LMCT and (b) modified LMCT: (1) C-F
3
, (2) C-F
2
, (3) C-F, (4) CO
3
, (5) C1s, (6) C-H (or C).
characteristics of untreated LMCT particles were changed when the
functional groups of F8261 were grafted. Fig. 7 displays the
photograph of test droplet on F8261 optimized LMCT specimen.
Obviously, the contact angle of F8261 optimized LMCT particles was
ꢀ
1
56 , representing a strong hydrophobic characteristic. As dis-
cussed above, the hydrophobic characteristic of F8261 modified
LMCT powders was contributed by the grafted functional groups of
-
(CH
2
)-(CF
2
)
5
-CF
3
which were from F8261. As we known, PTFE was
-CF - bonds. Accordingly, it was reasonable
also comprised of eCF
2
2
to predict that the modified LMCT powders and PTFE could be
recombined well.
After the modification process on LMCT powder, the surface of
modified LMCT powder possessed the similar molecular structures
2 2 2 2
(-CF -CF - bonds) as PTFE matrix. The eCF -CF - bonds on the
surface of modified LMCT powders could entangle with PTFE ma-
trix since similar molecular structures significantly promoted the
compatibility between modified LMCT and PTFE. To investigate the
interface characteristics between modified LMCT particles and PTFE
matrix, TEM images of F8261 modified PTFE/LMCTcomposites were
conducted and displayed in Fig. 8. As a comparison, the TEM image
of untreated LMCT filled PTFE composite was also attached in
Fig. 8(a). As marked, the dark colored solid was LMCT ceramic
particles, while the gray colloform was PTFE matrix. Fig. 8(a) shows
a bad connection between LMCT and PTFE, while Fig. 8(b) displays a
perfect adhesion. Particularly, as shown in Fig. 8(c), the surface of
LMCT particle was compactly surrounded by PTFE matrix. There
was not even a pore existed in the interface between LMCT and
PTFE matrix, representing a strong compatibility and adhesion. This
phenomenon directly proved that the modification process on
LMCT powders was successful and the compatibility between
inorganic LMCT ceramic powders and organic PTFE matrix was
greatly promoted.
The cross sectional SEM images of PTFE based and LMCT filled
composites were shown in Fig. 9. The weight fraction of LMCT
changed from 30 wt % to 70 wt % with a step size of 10 wt %. It can
be seen that all specimens exhibited two-phase structure (PTFE and
LMCT). We could demonstrate from Fig. 9(a)e9(e) that the PTFE
based substrates filled with various content of modified LMCT
powders exhibited compact and compatible interfacial micro-
structure. On the contrary, untreated LMCT filled PTFE composite
displayed loose and porous structure. The distinct comparison
indicated that the F8261 behaved as a medium between inorganic
LMCT and organic PTFE which improved the bonding at the inter-
face region. PTFE completely covered the surface of LMCT particles,
especially at low LMCT loading amount. The microstructure of the
composites became porous and abrupt with further increasing of
LMCT contents, and it was resulted from the lack of PTFE to make
Fig. 5. FTIR spectra of (a) untreated LMCT and (b) modified LMCT powder.
Ti bond confirmed the interaction between F8261 and LMCT again.
As shown in Fig. 6, TEM images were conducted to observe the
morphology of both untreated LMCT particles and silane coupling
agent modified LMCT particles. Fig. 6(a) depicts the TEM image of
untreated LMCT particles, while Fig. 6(b and c) present the TEM
images of F8261 modified LMCT particles. It was obvious that the
TEM image of the untreated LMCT particles was solid and brunet,
while the images of optimized LMCT particles were hazy and light-
colored. The clear distinctions in TEM images were due to that the
functional groups of F8261 had transferred and covered the
ambient surface of LMCT particles via optimizing procedure [25].
Translucent and hazy films were observed on F8261 modified LMCT
powders, as shown in Fig. 6(b and c), which were mainly related to
the organic functional groups came from coupling agent. Especially,
as shown in Fig. 6(c), the section marked by red oval was corre-
sponded to the entanglement between different optimized LMCT
powders. The observed entanglement was primarily contributed by
-
2 2 5 3
(CH )-(CF ) -CF chains grafted on the ambient of modified LMCT
powders.
In this section, the picture of Contact angle was measured to
present the hydrophobic nature of F8261 modified LMCT particles.
Due to the strong hydrophilic nature, untreated LMCT powders
ꢀ
possessed contact angle of near 0 which was consistent with most
inorganic ceramics. In other words, the water droplets released
during the test were entirely absorbed by untreated LMCT powders
when it contacted the samples. However, the hydrophilic

F. Luo et al. / Journal of Alloys and Compounds 803 (2019) 145e152
149
Fig. 6. TEM of LMCT particles (a) untreated LMCT and (b, c) F8261 modified LMCT.
polymer based and ceramic filled substrates. The equations of
different models were as follows:
Maxwell-Wagner equation [26]:
ꢀ
ꢁ
2
ε
c
þ ε þ 2Vf εf ꢂ ε
c
f
ε
¼ ε
c
ꢀ
ꢁ
(1)
eff
2
ε
c
þ ε ꢂ Vf εf ꢂ ε
c
f
Lichtenecker equation [27]:
ꢀ
ꢁ
ln εeff ¼ V logε þ 1 ꢂ V logε
c
(2)
(3)
f
f
f
Modified Lichtenecker equation [28]:
ꢂ ꢃ
ε
ε
f
log εeff ¼ V ð1 ꢂ kÞlog
þ logε
c
f
c
Fig. 7. Contact angle of water droplet on the modified LMCT powders.
f c
where ε , ε and εeff were related to the permittivities of LMCT fillers,
sure the liquidity of the composite matrix.
PTFE and the composites, respectively. The parameter k in Eq. (3)
Fig. 10(a) depicts the change of dielectric constant and dielectric
loss of PTFE based and LMCT filled composites (around 10 GHz)
with increasing the weight fraction of LMCT. Obviously, the
dielectric constant of LMCT/PTFE composites was markedly
increased by the modification on LMCT particles, while the loss
was a fitting factor, and V represented the volume fraction of
LMCT. Among these assumptive models, it was common for re-
f
searchers to use the Lichtenecker model (Eq. (2)) to predict their
works, which deemed that the fillers were ideal spherical
morphology, and were uniformly dispersed into polymer matrix
with randomly orientation [17]. Concerning about Modified Lich-
tenecker model, as reported, k ¼ 0.3 was most optimal for ceramics
filled polymers composites. The model of Maxwell-Wagner was
usually functioned at low test frequency because the interfacial
polarization was prominent at this range.
Fig. 10(b) compared the measured permittivities of LMCT/PTFE
composites with theoretical equations mentioned above (around
10 GHz). The measured permittivities of untreated LMCT and opti-
mized LMCT filled composites showed the same trend as that of
theoretical values calculated from above discussed models. As
tangent (tan d) of the composite substrates showed the opposite
trend. The optimized dielectric properties of LMCT/PTFE compos-
ites were primarily attributed to the modified surface characteris-
tics and compatible connection within the composite matrix which
could also be evidenced from Fig. 9. Notably, the PTFE based com-
posite filled with 50 wt % optimized LMCT particles possessed
appropriate permittivity of 4.33 and extremely low loss tangent of
0
.001.
As reported in previous papers, numerous theoretic models
were performed to calculate and predict the dielectric constant of

150
F. Luo et al. / Journal of Alloys and Compounds 803 (2019) 145e152
Fig. 8. TEM image of (a) untreated LMCT filled PTFE composite, (b) modified LMCT filled PTFE composites and (c) enlargement of image (b).
Fig. 9. Cross sectional SEM images of LMCT filled PTFE composites (a) 30 wt % (b) 40 wt % (c) 50 wt % (d) 60 wt % (e) 70 wt % (f) untreated LMCT filled PTFE composite.

F. Luo et al. / Journal of Alloys and Compounds 803 (2019) 145e152
151
Fig. 10. (a) Variation of dielectric properties (at 10 GHz) with filler content for
Li
Li
Li
2
Mg2.88Ca0.12TiO
2
Mg2.88Ca0.12TiO
2
Mg2.88Ca0.12TiO
6
6
6
/PTFE composites fabricated using untreated and modified
Fig. 10(b) Comparison of measured dielectric constant of
/PTFE composites with various theoretical models.
.
shown in Fig. 10(b), the Modified Lichtenecker curve matched well
with untreated LMCT filled composites when the filling content of
LMCT were 30 wt %, 40 wt % and 50 wt %. Nevertheless, the
measured permittivities of untreated LMCT filled PTFE slowly
deviated from the values calculated from Modified Lichtenecker
equation when the weight fraction of untreated LMCT increased up
to 60 wt % and 70 wt %. This deviation was originated from the
worse dispersion of LMCT fillers and untight interfacial connection
between untreated LMCT particles and PTFE matrix. It is noted that
the measured data of the composites filled with F8261 modified
LMCT fitted well with Lichtenecker model when the filling contents
of modified LMCT were 30 wt %, 40 wt % and 50 wt %. However, as
the filling content increased up to 60 wt % and 70 wt %, Maxwell-
Wanger model displayed a better consistent with the dielectric
constant of F8261 modified LMCT/PTFE composites. Accordingly,
the dielectric constant of F8261 optimized LMCT/PTFE composites
matched well with Lichtenecker and Maxwell-Wanger model. This
was due to the improved surface characteristics of modified LMCT
fillers, which resulted in compact connection between LMCT par-
ticles and PTFE matrix.
Fig. 11. (a) Variation of dielectric constant with temperature for LMCT/PTFE compos-
ites. (b) Variation of dielectric constant with frequency for LMCT/PTFE composites.
[
29]. The dielectric constant of all specimens increased less than 2%
ꢀ
when the ambient temperature rose from 30 to 110 C. This phe-
nomenon implied that the permittivities of LMCT/PTFE will not be
disturbed by the change of ambient temperature in this range. The
frequency dependence of permittivities of LMCT/PTFE respected to
different weight fraction of LMCT was shown in Fig. 11(b). For all
specimens, the permittivities kept almost constant at the range
ꢀ
from 4 GHz to 18 GHz under a given temperature of 30 C.
Furthermore, the plots also displayed that the permittivities of
LMCT/PTFE composite substrates increased with the increasing of
LMCT filling content.
4. Conclusion
It is well known that the temperature and frequency depen-
dence of permittivities played an increasingly important role in the
development of microwave devices design. Fig. 11(a) displays the
temperature dependence of permittivities of LMCT/PTFE with the
LMCT was modified by F8261, and then loaded in PTFE sus-
pension using high-speed stirring. The surface characteristics of
untreated LMCT and F8261 optimized LMCT powders were studied
using XPS, FTIR spectra and TEM analyses. It is clear that the
functional groups of F8261 grafted and covered the surface of LMCT
particles, successfully. Microstructure and dielectric properties of
LMCT/PTFE composite substrates were investigated as the weight
fraction of LMCT increased from 30 to 70 wt %. The cross sectional
SEM morphologies displayed that the ceramic fillers uniformly
distributed in PTFE. The dielectric constant of PTFE based and LMCT
ꢀ
ambient temperature ranged from 30 to 110 C. The dielectric
constant of all the specimens in the study experienced a slight in-
crease with the increasing of temperature. It was due to that the
segmental mobility of PTFE in the matrix has been promoted by the
raise of temperature, which would improve the polarization of
LMCT thereby resulting in an increasing in the dielectric constant

152
F. Luo et al. / Journal of Alloys and Compounds 803 (2019) 145e152
filled composites increased more or less linearly with the
increasing of LMCT content. In addition, the dielectric constant
experienced a slight increase as the ambient temperature raised
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x
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Acknowledgement
4
[
[
3
This work was supported by National Natural Science Founda-
tion of China (Grant No. 51672038).
2
2
4
4
Appendix A. Supplementary data
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