Zn Al 2 O 4 and (0.79) Zn Al 2 O 4-(0.21) Mn 2 TiO 4 microwave dielectric ceramics prepared by hot pressing and spark plasma sintering

Article abstract of DOI:10.1111/j.1551-2916.2011.04907.x

Ceramics of ZnAl₂O₄ (ZA) and 0.79ZnAl ₂O₄-0.21Mn₂TiO₄ (ZAMT) were prepared by the mixed oxide route using hot pressing and spark plasma sintering (SPS) techniques. Sintering temperatures we

Full text of DOI:10.1111/j.1551-2916.2011.04907.x

J. Am. Ceram. Soc., 95 [3] 1023–1028 (2012)
DOI: 10.1111/j.1551-2916.2011.04907.x
©
2011 The American Ceramic Society
ournal
J
ZnAl O and (0.79)ZnAl O –(0.21)Mn TiO Microwave Dielectric Ceramics
2
4
2
4
2
4
Prepared by Hot Pressing and Spark Plasma Sintering
‡
‡
§
§,†,*
Israel Lorite, Miguel Angel Rodriguez, Feridoon Azough, Robert Freer,
and
‡
Jose Francisco Fernandez
‡
Instituto de Ceramica y Vidrio, CSIC, Madrid, Spain
´
§
School of Materials, Material Science Centre, University of Manchester, Manchester, M13 9PL, U.K.
6
Ceramics of ZnAl
(
pressing and spark plasma sintering (SPS) techniques. Sinter-
ing temperatures were 1300°C in all cases; hot-pressed samples
were held at peak temperature for 1 h, the SPS samples for
2
O
4
(ZA) and 0.79ZnAl
ZAMT) were prepared by the mixed oxide route using hot
2
O
4
–0.21Mn
2
TiO
4
s
f
~ ꢀ50 ppm/°C), the spinel MgAl
2
O
4
investigated at
= 8.8,
microwave frequencies in 2005 by Surendran et al. (e
r
7
Q xf ~ 68 900 GHz, s = ꢀ75 ppm/°C), the related zinc alu-
u
f
minate spinel ZnAl
2
O
4 r u
(e = 8.5, Q xf ~ 56 300 GHz, and
8
s
f
~ ꢀ79 ppm/°C) investigated first by Surendran et al., and
5 min. Sintered densities were at least 98% theoretical for the
SPS samples. ZnAl O prepared by SPS was single phase; all
Mg TiO (e = 14, Q xf = 150 000 GHz, and s = ꢀ50 ppm/
2
9
4
r
u
f
,10
°C).
component materials, which is significant and negative
(s
The poor temperature dependence of these single
2
4
other samples contained minor amounts of second phase. Grain
sizes for ZnAl were typically 200–500 nm, but 2–5 lm for
ZAMT. FTIR spectra for all the samples include Al–O bend
2
O
4
f
= ꢀ79 to ꢀ50 ppm/°C), prompted interest in two compo-
nent systems where a material having a positive temperature
dependence of resonant frequency was combined with one of
ꢀ
1
mode band at ~450 cm . The microwave dielectric properties
of the ZA-SPS (e = 8.7; Qxf = 57 000 GHz) were compara-
ble with conventionally produced samples; the dielectric proper-
r
the above materials. This began with the addition of TiO
ZnAl , such as in 0.83ZnAl –0.17TiO ceramic by
Surendran et al., yielding a two phase ceramic with dielec-
tric properties of e = 12.67, Q xf = 100 000 GHz, and
= 0.74 ppm/°C. For the related formulation of
2
to
2
O
4
2
O
4
2
8
ties of the ZAMT-SPS (e = 9.6; Qxf = 30 630 GHz) were
r
superior to conventionally produced ceramics. The SPS
approach yielded high density, high quality dielectric ceramics
at lower sintering temperatures and much reduced processing
times.
r
u
s
f
1
1
0.79ZnAl O –0.21TiO Lei et al.
obtained comparable
2
4
2
properties
of
er = 11.4,
Q xf = 71 810 GHz, and
u
s
f
= ꢀ0.5 ppm/°C). Generally the presence of the second
11
phase (rutile in such TiO -doped systems) reduces the Q xf
2
values and so there has been growing interest in two com-
ponent formulations leading to solid solution products. For
u
I. Introduction
ITH the continued expansion of microwave communi-
cation systems into frequency bands above 20 GHz
W
example in the system (1 ꢀ x)ZnAl
2
O
4
–xMg
et al. indicated a solid solution for the entire composition
range, with er of 7.90–8.56, maximum Q xf of
106 000 GHz and s
2 4
TiO , Zheng
1
2
there has been growing interest in millimeter-wave dielectric
ceramics for applications in fields including very small aper-
ture terminal (VSAT) systems, local multipoint distribution
a
u
1
f
in the range ꢀ73 to ꢀ63 ppm/°C. To
service (LMDS), and ultrahigh speed wireless local area net-
work (LAN). For these higher frequency bands the micro-
achieve high densities, sintering temperatures in the range
1550°C–1650°C were needed. From analysis of the infra-red
reflection spectra of the (1 ꢀ x)ZnAl O –xMg TiO ceramics
2
wave dielectric materials must exhibit (i) very high unloaded
quality (Q ) factor (described in terms of Q xf product,
2
4
2
4
1
2
u
u
and the predicted dielectric properties, Zheng et al. sug-
where f is the resonant frequency) to ensure a high level of
signal discrimination, and (ii) zero temperature coefficient of
gested that enhancement of the dielectric properties could be
achieved by improving the microstructures. Subsequently, Lei
3
,4
1,13,14
resonant frequency (τ
tional requirement for microwave dielectrics to have high rel-
ative permittivity (e ) for component miniaturization is less
f
) for temperature stability. The tradi-
et al.
investigated ZnAl
2
O
4
-based microwave dielectric
(where
–0.21Mg TiO
they reported solid solution over part of the composition
range and a very high Q xf value of 160 800 GHz, which
was higher than that achieved in either of the end members,
with s
ceramics prepared with additions of 2MO–TiO
M = Co, Mg, and Mn). For 0.79ZnAl
2
r
2
O
4
2
4
,
important at the higher frequencies since the component size
scales inversely with increasing frequency. Indeed the highest
u
u
Q xf values tend to be found in materials with relatively low
5
relative permittivities, typically <15. Utilizing materials with
f
of ꢀ65.3 ppm/°C. However, it was still necessary to
e < 15 at ultra high frequencies helps to shorten the signal
employ a high sintering temperature of 1550°C to achieve
95% theoretical density. In contrast, high density ceramics of
r
2
,5
transmission times and to minimize crosstalk.
In recent years, a range of low permittivity dielectric
materials have been investigated for microwave and millime-
ter-wave applications. These include single phase ceramic
2 4 2 4 2 4 2 4
0.79ZnAl O –0.21Co TiO and 0.79ZnAl O –0.21Mn TiO
were obtained at lower sintering temperatures (1500°C and
1400°C respectively), but they also exhibited lower Q xf val-
u
materials such as MgTiO
3
(e
r
= 17, Q
u
xf = 160 000 GHz,
ues of 94 000–147 000 GHz and 23 530 GHz, res-
pectively. The degradation in Q xf values of the
u
Mn-containing specimens was ascribed to the presence of a
1
,13
4
+
1
ZnMn
dependence of resonant frequency was similar for all the
.79ZnAl –0.21M TiO
ceramics, being ꢀ66.4, ꢀ65.3, and
63.1 ppm/°C for Co, Mg and Mn, respectively. To achieve
adequate temperature compensation, with s values close to
zero, required the further addition of positive s materials
3
O
7
glass phase and Mn
ions. The temperature
N. Alford—contributing editor
0
ꢀ
2
O
4
2
4
1
Manuscript No. 29983. Received July 12, 2011; approved September 12, 2011.
Member, The American Ceramic Society.
f
*
†
f
Author to whom correspondence should be addressed. e-mail: robert.freer@
manchester.ac.uk.
1
4,15
such as CaTiO or SrTiO .
3
3
1023

1
024
Journal of the American Ceramic Society—Lorite et al.
Vol. 95, No. 3
The spinel ZnAl
2
O
ponent of a family of low permittivity dielectrics,
4
is now well established as a key com-
Netherlands), employing CuKa1 radiation under the condi-
tions 50 kV and 40 mA. The samples were scanned at 0.03°
intervals of 2h in the range 20–100°, the scan rate was 0.01°
1
,5,8,9,11–15
with potential for microwave and millimeter-wave applica-
tions. A significant drawback in their exploitation is the poor
sinterability of ZnAl
ing temperatures and/or the incorporation of transition metal
oxides. The majority of studies to date of ZnAl O -based
materials have employed conventional sintering procedures,
although alternative approaches, including hot pressing (HP)
and spark plasma sintering (SPS), may offer benefits in terms
of higher densities and shorter sintering times. Both HP and
SPS techniques have been successfully applied to the process-
ing of dielectrics, yielding superior densities, good dielectric
properties, and shorter sintering times.
The primary objective of the present study is to investigate
the influence of pressure-based sintering methods (namely
ꢀ
1
2hs . Following identification of the peaks, lattice parameters
1
9
2
O
4
, necessitating the use of high sinter-
were determined using the Topas refinement program.
For microstructural examination, by scanning electron
microscopy, a Field Emission-SEM Hitachi-S4700 (Hitachi
High-Tech, Minato-ku, Tokyo, Japan) was employed. Sam-
ples were ground then polished down to 0.25 lm diamond
paste followed by polishing with OPS (colloidal silica suspen-
sion) for 5 h. Finally they were thermally etched: heated to
1100°C at a rate of 20°C/min, held for 10 min and cooled at
20°C/min.
2
4
1
6–18
Infra-Red spectra were collected from polished samples
(prepared as described above for electron microscopy) with a
Perkin Elmer Spectrum 100 Spectrometer (Waltham, MA) by
attenuated total reflectance (ATR) within the range 250–
2 4
HP and SPS) on the preparation of ZnAl O -based ceramics
and the resulting dielectric properties. We have specifically
chosen (i) the end member ZnAl O , as it is a primary com-
ponent in many systems and (ii) 0.79ZnAl
as a typical (but demanding) example of a solid solution sys-
ꢀ
1
800 cm . The relative permittivity (e ) and the unloaded
r
Q xf values were determined in the TE mode at ~5 GHz
011
2
4
u
2
0
2
O
4
–0.21Mn
2
TiO
4
by the Hakki and Coleman method.
tem. The properties of 0.79ZnAl O –0.21M TiO ceramics
2
4
2
4
III. Results and Discussion
prepared by conventional means are now well docu-
mented,
1
,12–14
and the Mn analog is challenging as it exhibits
quite modest dielectric properties and there are potential
complications from a glass phase and valency issues.
The benefits of SPS and HP processing over conventional
sintering are evident in Table I. With the exception of hot
pressed ZnAl O (ZA-HP), all the samples prepared by SPS
2
4
and HP exhibit densities of at least 98% theoretical, signifi-
cantly higher than equivalent compositions prepared by con-
II. Experimental Procedure
1,8,11,13
ventional routes.
An additional benefit of SPS
The starting materials were reagent-grade powders of ZnO
(
processing is the very short sintering time, 5 min at peak
temperature of 1300°C, compared to typically 3 h at 1550°C
for conventionally prepared ZA and 3 h at 1400°C for
2 3 2
99.5%), Al O (98.5%), TiO (99.5%), and MnO (99%)
supplied by Sigma Aldrich (Gillingham, U.K.). To prepare
ZnAl O , the ZnO and Al O powders were first mixed in
1
ZAMT. The low density for ZA-HP reflects the fact that the
2
4
2
3
a stoichiometric ratio and milled with zircona media in de-
ionized water for 2 h; the slurry was then dried at 80°C. The
resulting powder was pressed into pellets 25 mm thick and
1300°C temperature used in this study for all HP and SPS
samples (for comparison purposes) was not optimum for HP
ZA, due to its poor sinterability.
1
0 mm diameter, at 40 MPa.
4
The sintering of ZnAl O by (i) hot pressing (HP) and (ii)
Collected X-ray diffraction spectra for the SPS and HP
samples are presented in Fig. 1. It is clear that all exhibit the
cubic spinel structure and the data can be fitted to the model
of Fd3m space group for a normal spinel, but only the SPS
ZnAl O is a single phase product. Preliminary NMR data
2
spark plasma sintering (SPS) was performed in situ by heat
treatment of the pellets in a Dr. Sinter SPS-1050-CE (SPS
Syntex Inc., Osaka, Japan). The two sets of specimens are
denoted as ZA-HP and ZA-SPS, respectively. For HP the
temperature was increased to 850°C and held for 30 min,
then increased to1300°C and held for 1 h, and finally cooled
to room temperature. The load applied to the sample was
2
4
for both types of ZA suggested that there could be minor
inversion of the primary spinel phase, with up to 3% of diva-
lent ions on octahedral sites. This is consistent with the find-
2
1
ings of Cooley and Reed who reported typically 4%
inversion of ZnAl at room temperature. The X-ray dif-
fraction data for hot pressed ZnAl (ZA-HP) shows
(Fig. 1) traces of Al O , and the low density (86% theoreti-
4
0 MPa; all heating and cooling rates were 10°C/min. For
SPS, the ZnAl pellets were heated to 850°C and held for
min, then heated to 1300°C and held for 5 min, and finally
2 4
O
2
O
4
2 4
O
5
2
3
cooled to room temperature. The load applied to the samples
was 40 MPa; all heating and cooling rates were 250°C/min.
cal) probably reflects incomplete reaction, and certainly
incomplete sintering. The existence of alumina as a second
phase suggests that Zn has been lost during processing (as
reported earlier by van der Laag et al. ). The situation is
more complex for the ZAMT samples. For hot pressed
2 4 2 4
To prepare samples of 0.79ZnAl O –0.21Mn TiO
denoted as ZAMT-HP and ZAMT-SPS for HP and SPS,
2
2
(
respectively), the ZnAl
wet milling ZnO and Al
2
O
4
powder was firstly obtained by
(as described earlier) and by cal-
2
O
3
2 4 2 4
0.79ZnAl O –0.21Mn TiO (ZAMT-HP) there are two addi-
cining at 1200°C for 2 h. Once the ZnAl O was obtained
tional phases Zn0.76Mn0.24O and MnTiO , confirming that
2
4
3
(
confirmed by X-ray diffraction), appropriate amounts of
MnO and TiO were added, to yield the formulation
.79ZnAl –0.21Mn TiO ; this was wet milled for 3 h with
Mn in both the matrix and secondary phases is of valency
two. The X-ray diffraction spectrum for ZAMT prepared by
SPS exhibits significantly less secondary phase (in part
2
0
2
O
4
2
4
de-ionized water and zirconia media and dried. The raw
powders of ZAMT were pressed into pellets, using the same
conditions as for the ZA powders. Finally, the ZAMT pellets
were sintered, using the same heat treatment as previously
described for the ZA-HP and ZA-SPS materials. The pro-
cessing of both sets of samples was performed under an Ar
atmosphere. All specimens were subjected to additional heat
treatment at 1100°C for 24 h to ensure that the samples were
oxidized and to remove any traces of carbon that may have
been picked up from the dies during HP and SPS processing.
Specimen densities were determined by the Archimedes
method. Phase analysis and crystal structures were examined
by X-ray diffraction. This was undertaken using a Philips
Analytical, X’pert-MPD (PANanalytical B.V., Almelo, The
Table I. Densities of ZnAl O (ZA) and 0.79ZnAl O –
2
4
2
4
0.21Mn TiO (ZAMT) Ceramics
2
4
Composition
Relative density (%)
Processing route
Study
ZA-SPS
ZA-HP
ZA
ZAMT-SPS
ZAMT-HP
ZAMT
99
85
93
99
98
96
SPS
HP
Conv.
SPS
HP
TS
TS
13,22
TS
TS
1
Conv.
Conv., conventional TS, this study.

March 2012
ZAMT Preparation by HP and SPS Techniques
Figures 2(a) and (b) show SEM images of ZnAl
1025
pre-
2
O
4
pared by HP and SPS techniques, respectively. The high level
of porosity in the ZA-HP sample is clearly evident, with
angular or irregularly shaped grains typically 0.5 lm in size.
The enlarged view of a matrix grain [shown as inset in
Fig. 2(a)] provides evidence of growth banding. There is no
indication of the second phase alumina in SEM images. The
microstructure of single phase ZA-SPS is presented in
Fig. 2(b). The grains are much more rounded than those in
Fig. 2(a), and the smaller grain size, typically 200–400 nm,
reflects the short time (5 min) that the samples spent at maxi-
mum temperature. There are occasional grains up to 1 lm in
size, but they are comparatively rare. In contrast, the grain
sizes are much larger in the ZAMT samples and second
phases are evident [for example the grains indicated by
arrows in Figs. 2(c) and (d)]. In spite of using the same sin-
2 4
tering temperature and time as for ZnAl O samples, the
grain sizes in ZAMT are up to 10 times larger than those for
ZA; typically 3–6 lm for the matrix grains and 2–4 lm for
second phases in ZAMT-HP, and 3–4 lm for matrix grains
in ZAMT-SPS. However, the grain sizes in both sets of sam-
ples are much smaller (due to the application of pressure)
than those prepared by conventional methods (typically 10–
2 4 2 4
Fig. 1. X-ray diffraction spectra of ZnAl O (ZA) and 0.79ZnAl O
2 4
–0.21Mn TiO (ZAMT) ceramics processed by HP and SPS.
because of the very short sintering time); there is a small
amount of MnTiO3.and Al , but no Zn0.76Mn0.24O, sug-
gesting some loss of Zn. This contrasts with the work of Lei
1
2
O
3
14 lm; Lei et al. ). The grain growth in the ZAMT samples
and the formation of second phases is almost certainly due
to the development of liquid phases during sintering. For
example in the system Zn–Ti–O there are eutectics at 800°C
1
3 7
et al. who found grains of ZnMn O ; the latter was inter-
preted as a glass phase, responsible for lowering the sintering
temperature by 200°C. The presence of ZnMn O in the sam-
2
5
3
4+
7
2 4
and 945°C (Yang and Swisher ) and formation of Zn TiO
1
ples of Lei et al. indicates manganese as Mn . We did not
and ZnTiO ; the system Mn–Ti–O has eutectics at ~890°C
3
detect any evidence of ZnMn as the post-sintering oxygen
7
3
O
and 1015°C leading to the formation of MnTiO (Chufarov
3
2
6
anneal at 1100°C ensured that the manganese was in a 2+
state.
Calculated lattice parameters are listed in Table II. The
et al. ). The dominant second phases in ZAMT are MnTiO
3
and Zn0.76Mn0.24O (or Zn MnO ), consistent with phases
3
4
2
5,26
reported in the published phase diagrams.
In general the
experimental data for ZnAl
SPS routes (lattice parameter 8.0848 A), are broadly compa-
rable with extensive published data for conventionally pre-
2
O
4
, produced by both HP and
˚
use of SPS to prepare ZAMT (this investigation) led to the
development of a more uniform grain structure in ZAMT, a
more uniform grain size distribution and less second phase
[compared to the ZAMT-HP samples; Figs. 2(c) and (d)].
The FTIR spectra for the four types of samples are shown
in Fig. 3. There are clear similarities and differences between
them. It was impossible to collect good quality data below
2
2
pared samples (for example van der Laag et al.,
et al. ). In contrast, there is very limited published crystallo-
Lei
13
graphic data for Mn TiO -based compounds, even including
2
4
the end member. Our lattice parameter for ZAMT-SPS is
˚
.2043 A, somewhat less than the value of 8.3269 A reported
˚
ꢀ1
8
by Lei et al. for 0.79ZnAl
500 cm for ZA-HP; this probably reflects sample quality
and the high porosity. For ZnAl O there is minimal inver-
1
2
O
4
–0.21Mn
2
TiO
4
. The present
2
4
authors have been unable to locate any other data for the
same composition for direct comparison, but estimates are
sion of the spinel structure, with the majority of divalent
cations (Zn) in tetrahedral sites and the majority of trivalent
cations (Al) in octahedral sites. Chopelas and Hofmeister
2
7
possible based on end member compositions. Lattice parame-
˚
ters for ZnAl
(
2
O
4
, Mg
2
TiO
4
, and Mn
2
TiO
4
are 8.048 A
Table II), 8.4376 A (Weschler and von Dreele ), and
examined Raman and IR spectra for a range of aluminate
spinels including MgAl O , FeAl O , and ZnAl O . In all
2
3
˚
2
4
2
4
2
4
2
.627 A (Lambert et al. ), respectively. Assuming a simple
4
ꢀ1
˚
8
linear variation in lattice parameters between the end mem-
cases the band at 440–530 cm was the most intense; from
classical dispersion analysis the calculated value of this band
2 4
for ZnAl O was 440 cm . More recently, Zheng et al.
˚
ꢀ1
12
ber values yields 8.159 A for 0.79ZnAl O –0.21Mg TiO and
2
4
2
4
˚
8
for the Mg-containing compound is similar to the lattice
.169 A for 0.79ZnAl O –0.21Mn TiO . The calculated value
noted that on the basis of a simple oscillator model, for a
spinel structure with space group Fd3m, bands for four
vibration modes are observed in IR spectra: (i) Zn–O bend
2
4
2
4
1
parameter (8.207 A) reported by Lei et al. for that composi-
˚
ꢀ
1
tion. The calculated value for 0.79ZnAl O –0.21Mn TiO is
mode at 200–400 cm , (ii) Zn–O stretch mode at 550–
2
4
2
4
ꢀ
1
ꢀ1
much closer to the lattice parameters obtained for our SPS
590 cm , (iii) Al–O bend mode at 470–500 cm , (iv) Al–O
stretch mode at 640–670 cm . On the basis of a Kramers–
Kronig analysis of the IR reflectance data and calculating
˚
sample (8.2043 A) and our HP sample (8.180 A) than the
˚
ꢀ1
1
value of 8.3269 A reported by Lei et al. for ZAMT;
˚
additional crystallographic investigations of this system are
desirable.
real and imaginary parts of the dielectric functions for
(Mg1
1
2
Zn )Al O ceramics Zheng et al. concluded that
x 2 4
ꢀ x
dielectric loss in the system is caused mainly by the first two
vibration modes, suggesting that interactions between A-site
divalent cations and oxygen ions have the greatest effect on
extrinsic dielectric loss. However, their tabulated data for
Table II. Lattice Parameters for ZnAl
2
O
4
(ZA) and
0.79ZnAl
O
2 4
–0.21Mn
2
TiO (ZAMT) Ceramics
4
˚
Lattice parameter (A)
˚
3
Cell volume (A )
Composition
Study
(
with composition, but that in the majority of cases the band
Mg1 ꢀ xZn
x 2 4
)Al O indicate that the dominant modes vary
ZA-SPS
ZA-HP
ZA
ZAMT-SPS
ZAMT-HP
ZAMT
8.0848
8.0848
8.0811
8.2043
8.1800
8.3269
528.45
528.45
527.72
552.24
547.34
577.36
TS
TS
ꢀ1
27
at 470–500 cm
is strongest (in agreement with Chopelas
ꢀ1
and Hofmeister ), with either the 200–400 cm or the 640–
13,22
TS
TS
ꢀ1
6
70 cm band being second strongest. This implies that the
Al–O bend mode is in fact dominant, with the Zn–O bend
mode and Al–O stretch mode making significant contribu-
tions. Thus, interactions between octahedral site ions and
oxygen also appear to be important. Our spectra for
1
TS, this study.

1
026
Journal of the American Ceramic Society—Lorite et al.
Vol. 95, No. 3
(
a)
(b)
(
c)
(d)
Fig. 2. Scanning electron micrographs of sintered ceramics: (a) ZA-HP; (b) ZA-SPS; (c) ZAMT-HP; (d) ZAMT-SPS.
bands depend on cation mass, particularly of tetrahedral site
ions. Hence, the band frequencies for (Fe0.58Mg0.42)Al
effective atomic mass for tetrahedral site ions = 42.5) were
lower than for ZnAl O (where the atomic mass was 65.4 for
2 4
O
(
2
4
Zn in the tetrahedral sites). They noted that the wave num-
bers for the three highest modes decreased at less than half
the rate of the lowest wave number modes with decreasing
tetrahedral cation mass. On this basis we would expect the IR
bands for 0.79ZnAl
numbers than those for ZnAl
2
O
4
–0.21Mg
2 4
TiO to be at lower wave
2 4
O , since Zn (mass 65.4) is par-
tially replaced by Mn (mass 54.4). However, with only 21%
Mn replacement for Zn, the effective mass difference is rela-
tively modest so the predicted changes in IR spectra are less
2
7
than those observed by Chopelas and Hofmeister between
ZnAl and (Fe0.58Mg0.42)Al . Indeed, there are virtually
no differences between the bands in ZA and ZAMT at the
2
O
4
2 4
O
ꢀ
1
higher wave numbers (above 500 cm in Fig. 3), but there
are small differences at lower wave numbers, (for example at
Fig. 3. FTIR spectra of sintered ceramics: (a) ZA-HP; (b) ZA-SPS;
c) ZAMT-HP; (d) ZAMT-SPS.
ꢀ1
4
ble to comment on the behavior of the very low wave number
51 and 408 cm ) just as predicted. Whereas it is not possi-
(
ꢀ
1
mode in ZAMT (around 220 cm ) it is clear that there are
significant differences in peak shape and width between the
ꢀ
1
ZnAl O (Fig. 3) are broadly in agreement with the findings
2
spectra for ZA and ZAMT below 500 cm . This almost cer-
tainly reflects the effect of introducing Mn and Ti into
4
1
2,27
of earlier investigators,
with a major peak in ZA-SPS at
ꢀ
1
4
51 cm
The structure of Mn
spinel (space group P4 22 or P4 22), described as Mn(Ti,Mn)
.
2 4
ZnAl O , but may in part reflect the nature and quality of the
2
4
TiO is essentially that of a normal
samples.
The microwave dielectric properties of the ZA and ZAMT
materials are summarized in Table III. The low density of
the ZA-HP ceramics (86%) prevented resonance measure-
ments; for this reason data are not included in Table III. For
the ZnAl O ceramics prepared by SPS, the dielectric proper-
1
+
3
2
4
O with half the Mn ions on A sites and the remainder on
4
+ 28
the B sites in 1:1 ratio with Ti
.
Thus adding Mn
2
TiO
will share the A sites
~21%:79%, ignoring minor amount of Al due to structural
4
to
2
+
2+
ZnAl O means that Mn
and Zn
2
4
(
2
4
2
+
4+
inversion) and the B sites will be occupied by Mn , Ti
,
ties are broadly comparable with data reported by other
workers.
3
+
8,12
and Al
size between the Zn
sites (0.600 and 0.660 A, respectively), and Al , Ti , and
(~10.5%, 10.5%, 79%). With differences in ionic
Porosity is probably the single, most important
factor, controlling relative permittivity in pure, single phase
materials, explaining why the high density ZA-SPS (99% the-
2
+
2+
and Mn
on the 4-coordination A
3
+
4+
˚
2
+
Mn on the 6-coordination B sites (0.535, 0.605, and
is replaced
oretical) exhibits higher e (8.7) than the ZA ceramics pre-
r
2
.67 A), the lattice expands when 21% ZnAl
9
8,12
˚
0
by Mn TiO as noted above (increase from 8.048 to 8.204 A).
2
O
4
pared by other methods.
From an analysis of the FTIR
1
2
˚
spectra of ZnAl O –MgAl O , Zheng et al. predicted the
2
4
2
4
2
4
For the spectra there are changes due to mass differences of
relative permittivity of intrinsic ZA to be 9.3. It will be inter-
esting to see whether or not values close to this can be
achieved in single crystal ZnAl O . In contrast to the simple
2 4
2
7
the ions. For example, Chopelas and Hofmeister showed
that for MgAl , (Fe,Mg)Al , and ZnAl the major IR
2
O
4
2
O
4
2
O
4

March 2012
ZAMT Preparation by HP and SPS Techniques
1027
Table III. Microwave Dielectric Properties of ZnAl
O
2 4
(ZA)
ence of three different minor phases in the ZAMT-HP
(Fig. 1) and their presence to a significant fraction of the
microstructure (Fig. 2), will certainly contribute to a reduc-
and 0.79ZnAl O –0.21Mn TiO (ZAMT) Ceramics
2
4
2
4
Sintering
temperature,
T (°C)
tion in Q xf value. In contrast, the high density SPS-pre-
u
Qxf
τ
f
pared sample contains no more than about 5% of second
phase, and a much more uniform microstructure (Fig. 2),
leading to an enhancement of dielectric Q value. As it was
already known that both ZA and ZAMT exhibit poor tem-
perature dependence characteristics (temperature dependence
Composition
e
r
(GHz)
(ppm/°C)
Study
ZA-SPS
ZA-HP
ZA (C)
ZA (C)
ZAMT-SPS
ZAMT-HP
ZAMT (C)
1300
1300
1500?
1700?
1300
1300
1400
8.7
—
8.5
7.9
9.6
8.9
9.7
57 000
—
56 300
82 000
30 630
24 104
23 530
TS
TS
8
12
TS
TS
1
ꢀ79
ꢀ63
1
,8,12–14
of resonant frequency τ
ues were not assessed for the present samples. Indeed to
achieve near zero τ values in ZnAl –M TiO based sys-
tems requires the addition of a third component such as
f
values ~ ꢀ70 ppm/°C)
f
τ val-
f
2
O
4
2
4
ꢀ63
1
5
SrTiO
3
.
From the collected data in Table III it can be seen
TS, this study.
that processing ZnAl O and 0.79ZnAl O –0.21Mn TiO by
2
4
2
4
2
4
SPS techniques yielded high quality materials, exhibiting
good dielectric properties. The Q values obtained by SPS
processing are (at present at least) equivalent to those
obtained by traditional means. In addition, increasing the
dependence of permittivity on porosity, the dielectric Q
values are sensitive to impurities, defects, second phases and,
to a lesser extent, sample porosity. The unloaded Q xf value
for the high density ZA-SPS (57 000 GHz) has probably
been depressed because of the reducing atmosphere during
u
xf
fraction of Mn
mittivity, but reduce the Q
2
TiO
4
in ZAMT will increase the relative per-
xf value.
u
u
sintering. The Q
reported by Surendran et al., but less than that reported by
Zheng et al., both groups employed significantly higher sin-
tering temperatures and longer sintering times than for the
SPS-processed ceramics.
u
xf value is, however, very similar to that
IV. Conclusions
8
12
ZnAl O -based ceramics generally exhibit poor sinterability,
2
4
requiring high temperatures. The SPS approach to processing
ZnAl and 0.79ZnAl –0.21Mn TiO yielded high den-
2
O
4
2
O
4
2
4
sity products (at least 98% theoretical) with reduced sintering
temperatures (1300°C), and no more than 5 min at peak tem-
perature. Microstructures of ceramics produced by SPS were
more uniform and grain sizes smaller (200–400 nm for ZA
and 3–4 lm for ZAMT) than hot pressed or conventionally
processed materials. The shorter processing times for SPS
also minimized second phase development; ZA-SPS was sin-
Relative permittivities in the ZAMT system are expected
to scale according to the amounts of the constituent compo-
nents ZnAl
2 4 2 4
O and Mn TiO , which is 79:21 for the present
1
materials. Lei et al. noted that the Clausius–Mossotti equa-
3
0
tion can be used to calculate the relative permittivity of a
compound via:
ð3Vm þ 8paDÞ
gle phase, ZAMT-SPS contained ~5% of MnTiO (confirm-
3
ing Mn in divalent form) and a trace of Al O . The hot
3
er ¼
(1)
2
ð3Vm ꢀ 4paDÞ
pressed products contained significantly more second phases.
The normal spinel structure of ZA exhibits limited inversion,
with about 3% of divalent ions on octahedral sites; the lat-
tice expands as Mn and Ti are introduced into the lattice
´
3
˚
where Vm is molar volume (A ) and a is dielectric polariz-
ability. To a first approximation, Eq. (1) implies that e
D
r
is
directly proportional to a and inversely proportional to V .
D
m
1
˚
Lei et al. therefore anticipated that the relative permittivity
of a range of 0.79ZnAl O –0.21M TiO compounds (where
(8.048–8.204 A).
The IR spectra for ZA and ZAMT contain four major
2
4
2
4
1
peaks; Zheng et al. proposed that dielectric loss is caused
2
M = Mg, Co, and Mn) would vary directly with dielectric
polarizability. However, their experimental data for the three
primarily by interactions between divalent A-site cations and
compounds did not follow the predicted trend, with e
r
values
oxygen. Even with only 21% replacement of ZnAl
Mn TiO in ZAMT, the introduction of Mn ions in place of
Zn (with half the Mn and Ti substituting for Al on the B sites)
has a significant effect on the lattice vibrations, which com-
bined with the presence of second phase grains and a more
irregular microstructure leads to higher dielectric losses in
O by
2 4
showing comparatively little variation (9.6–9.9). It is clear
2
4
1
from the work of Lei et al. that addition of M
pounds (where M = Mg, Co, or Mn) to ZnAl
increases the relative permittivity. For the present ZAMT
2
TiO
O directly
4
4
com-
2
materials, the relative permittivity of 8.7 for ZnAl O
2
4
increased to 8.9 for ZAMT-HP, and to 9.6 for the higher
density ZAMT-SPS. The presence of the second phase
MnTiO3 in the ZAMT-SPS samples is expected to increase
u u
ZAMT: Q xf = 57 000 GHz in ZA-SPS; Q xf = 30 630 GHz
in ZAMT-SPS. For ceramics of fixed composition, porosity
has the greatest effect on relative permittivity; the 99% dense
the measured relative permittivity as the e
~
r
of MnTiO
22. However, as the amount of the second phase is proba-
bly no more than 5% (Fig. 1), the effect of the MnTiO will
be quite limited. From the relative permittivity data for
ZnAl and 0.79ZnAl –0.21Mn TiO (Table III), the rel-
TiO is estimated to
3
is
SPS samples exhibit e
r
values of 8.7 (ZA) and 9.6 (ZAMT). A
31
linear extrapolation of the data suggests that the relative per-
mittivity of Mn TiO
2 4
will be ~13.
3
2
O
4
2
O
4
2
4
References
ative permittivity for end member Mn
2
4
1
be ~13.0
W. Lei, W.-Z. Lu, J.-H. Zhu, F. Liang, and D. Liu, “Modification of
-Based Low Permittivity Microwave Dielectric Ceramics by Adding
MO-TiO (M=Co, Mg, and Mn),” J. Am. Ceram. Soc., 91, 1958–61
2008).
2 4
ZnAl O
Whereas the Q xf value for the hot pressed ZAMT sample
u
2
2
(
24 104 GHz) is very close to that reported by Lei et al. for
(
2
conventionally processed ZAMT (23 530 GHz), the Q
value for the SPS-processed ZAMT is significantly higher at
0 634 GHz (Table III). It is apparent from Fig. 3 that there
u
xf
H. Ohsato, F. Azough, E. Koga, I. Kagomiya, K. Kakimoto, and R. Freer,
3
“High Symmetry Brings High Q Instead of Ordering in Ba(Zn1/3Nb2/3)O by
HRTEM Study,” Ceram. Trans., 216, 129–36 (2010).
T. Tsunooka, M. Androu, Y. Higashida, H. Sugiura, and H. Ohsato,
3
3
are differences between the IR spectra for the two types of
ZAMT, with higher frequency peaks in ZAMT-SPS (higher
Q xf value) being more distinct than those in ZAMT-HP
“
Effects of TiO
2
on Sinterability and Dielectric Properties of High-Q Forsterite
Ceramics,” J. Eur. Ceram. Soc., 23, 2573–8 (2003).
J. C. Kim, M.-H. Kim, J.-B. Lim, S. Nahm, J.-H. Paik, and J.-H. Kim,
4
u
“
3 5
Synthesis and Microwave Dielectric Properties of Re Ga O12 (Re: Nd, Sm,
(
lower Q
expected to be less than extrinsic losses in both types of
ZAMT, and the resulting dielectric loss, and measured Q xf
values will be more sensitive to extrinsic factors. The pres-
u
xf value). Nevertheless, IR phonon losses are
Eu, Dy, Yb, and Y) Ceramics,” J. Am. Ceram. Soc., 90 [2] 641–4 (2007).
C.-L. Huang, T.-J. Yang, and C.-C. Huang, “Low Dielectric Loss Ceramics
5
u
in the ZnAl
24 (2009).
2 4 2
O -TiO System as a Compensator,” J. Am. Ceram. Soc., 92, 119–

1
028
Journal of the American Ceramic Society—Lorite et al.
Vol. 95, No. 3
6
19
K. Wakino, “Recent Developments of Dielectric Resonator Materials in
Japan,” Ferroelectrics, 91, 69–86 (1989).
K. P. Surendran, P. V. Bijumon, P. Mohanan, and M. T. Sebastian, “(1-x)
Bruker UK Limited, TOPAS Software (Ver. 2.1). Bruker AXS Ltd., Cov-
entry, U.K., 2003.
B. W. Hakki and P. D. Coleman, “A Dielectric Resonant Method of Mea-
7
20
MgAl
2
O
4
–XTiO
2
Dielectrics for Microwave and Millimeter Wave Applica-
suring Inductive Capacitance in the Millimeter Range,” IRE Trans. Microwave
Theory Tech., 8, 402–10 (1960).
R. F. Cooley and J. S. Reed, “Equilibrium Cation Distribution in
tions,” Appl. Phys. A, 81, 823–6 (2005).
K. P. Surendran, N. Santha, P. Mohanan, and M. T. Sebastian, “Tempera-
8
21
ture Stable Low Loss Ceramic Dielectrics in (1-x)ZnAl
2
O
4
–XTiO
2
System for
NiAl
(1972).
2 4 2 4 2 4
O , CuAl O , and ZnAl O Spinels,” J. Am. Ceram. Soc., 55, 395–8
Microwave Substrate Applications,” Eur. Phys. J. B, 41, 301–6 (2004).
C. W. Zheng, S. Y. Wu, X. M. Chen, and K. X. Song, “Modification of
9
22
N. J. van der Laag, M. D. Snel, P. C. M. M. Magusin, and G. de With,
“Structural, Elastic, Thermophysical and Dielectric Properties of Zinc Alumi-
MgAl
2 4
O Microwave Dielectric Ceramics by Zn Substitution,” J. Am. Ceram.
Soc., 90, 1483–6 (2007).
nate (ZnAl
2
O
4
),” J. Eur. Ceram. Soc., 24, 2417–24 (2004).
B. A. Wechsler and R. B. von Dreele, “Structural Refinement of Mg
MgTiO and MgTi by Time-of-Flight Neutron Powder Diffraction,” Acta
Cryst. B, 45, 542–9 (1989).
1
0
23
A. Belous, O. Ovchar, D. Durilin, M. M. Krzmanc, M. Valant, and D.
Suvorov, “High-Q Microwave Dielectric Materials Based on the Spinel
MgAl ,” J. Am. Ceram. Soc., 89, 3441–5 (2006).
W. Lei, W. Z. Lu, J. H. Zhu, and X. H. Wang, “Microwave Dielectric
Properties of ZnAl –TiO Spinel-Based Composites,” Mater. Lett., 61, 4066
9 (2007).
2 4
TiO ,
3
2 5
O
2 4
O
1
1
24
P. M. Lambert, “Magnetism and Superconductivity in the Spinel System
2
+
2+
2
O
4
2
Li1-xMn Ti
2
O
4
, Li1-xMg Ti
J. Yang and J. H. Swisher, “The Phase Stabilization of Zn
Characteract., 37, 153–9 (1996).
2
O
4
,” J. Solid State Chem., 75, 332–46 (1988).
2
5
–
2 3 8
Ti O ,” Mater.
1
2
C. W. Zheng, X. Z. Fan, and X. M. Chen, “Analysis of Infrared Reflec-
tion Spectra of (Mg1-xZn )Al Microwave Dielectric Ceramics,” J. Am.
Ceram. Soc., 91, 490–3 (2008).
2
6
x
2
O
4
G. I. Chufarov, A. M. Yankin, V. P. Demin, Y. V. Golikov, and V. F.
Balakirov, “Phase Relations in the Mn–Ti–O System in Air,” Zh. Fiz. Chem.,
60, 863–6 (1986).
A. Chopelas and A. M. Hofmeister, “Vibrational Spectroscopy of Alumi-
1
3
W. Lei, W.-Z. Lu, X.-H. Wang, F. Liang, and J. Wang, “Phase Composi-
tion and Microwave Dielectric Properties of ZnAl -Co TiO Low-Permittiv-
ity Ceramics With High Quality Factor,” J. Am. Ceram. Soc., 94, 20–3 (2011).
2
7
2
O
4
2
4
2 4
nate Spinels at 1 atm and of MgAl O to Over 200 Kbar,” Phys.Chem. Mins.,
1
4
W. Lei, W.-Z. Lu, D. Liu, and J.-H. Zhu, “Phase Evolution and Micro-
Ceramics,” J. Am.
18, 279–93 (1991).
E. F. Bertaut and E. H. Vincent, “Etude par Diffraction Neutronique de
2
8
wave Dielectric Properties of (1-x)ZnAl
2
O
4
-XMg
2
TiO
4
Ceram. Soc., 92, 105–9 (2009).
X. Wang, W. Lei, and W. Lu, “Novel ZnAl O -Based Microwave Dielec-
tric Ceramics with Machinable Property and its Application for GPS
la Forme Ordonnee de L’Orthotitanate de Manganese – Structure Crystalline
et Structure Magnetique,” Solid State Comm., 6, 269–75 (1966).
R. D. Shannon, “Revised Effective Ionic Radii and Systematic Studies of
1
5
2 4
2
9
Antenna,” Ferroelectrics, 388, 80–7 (2009).
S. Duguey, R. Lebourgeois, J. P. Ganne, and J. M. Heintz, “High Fre-
Interatomic Distances in Halides and Chalcogenides,” Acta Cryst., 32, 751–67
(1976).
1
6
3
0
quency Dielectric Properties of Hot-Pressed Ag(Nb
J.Eur. Ceram. Soc., 27, 1087–91 (2007).
x
Ta1-x)O
3
Solid Solutions,”
R. D. Shannon, “Dielectric Polarizabilities of Ions in Oxides and Fluo-
rides,” J. Appl. Phys., 73, 348–66 (1993).
1
7
31
H. Xue, Z. X. Xiong, and H. P. Zhou, “The Structure and Dielectric Tun-
able Properties of Fine-Grained Ba0.6Sr0.4TiO Ceramics Prepared by Spark
Plasma Sintering,” J. Am. Ceram. Soc., 90, 2653–6 (2007).
A. Tkach, P. Vilarinho, and A. L. Kholkin, “Structure–Microstructure–
Dielectric Tunability Relationship in Mn-Doped Strontium Titanate Ceram-
3
ics,” Acta Mater., 53, 5061–9 (2005).
h
1
8
H. Ning, H. Yan, and M. J. Reece, “Piezoelectric Strontium Niobate and
Calcium Niobate Ceramics with Super-High Curie Points,” J. Am. Ceram.
Soc., 93, 1409–13 (2010).