International Centre for Materials Physics, Chinese Academy of Sciences

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Nov 15, 2013 (3 years and 4 months ago)

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Summer School of Advanced Functional Materials 20
1
1

International Centre for Materials Physics, Chinese Academy of Sciences

ABSTRACT
S




Multiferroic Vortices and Graph Theory

S
-
W. Cheong

Rutgers Center for Emergent Materials

Rutgers University, Piscataway
, NJ 08854

sangc@physics.rutgers.edu


The fascinating concept of topological defects permeates ubiquitously our
understanding of the early
-
stage universe, hurricanes, quantum matters such as superfluids and
su
perconductors, and also technological materials such as liquid crystals and magnets.
Large
-
scale spatial configurations of these topological defects have been investigated only in a
limited degree. Exceptions include the cases of supercurrent vortices or l
iquid crystals, but they
tend to exhibit either trivial or rather
-
irregular configurations.

Hexagonal REMnO
3

(RE= rare earths) with RE=Ho
-
Lu, Y, and Sc, is an improper
ferroelectric where the size mismatch between RE and Mn induces a trimerization
-
type
str
uctural phase transition, and this structural transition leads to three structural domains, each
of which can support two directions of ferroelectric polarization. We reported

that domains in
h
-
REMnO
3

meet in cloverleaf arrangements that cycle through all
six domain configurations,
Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the
cycle of domain configurations is reversed. Vortices and antivortices are topological defects:
even in a strong electric field they won’
t annihilate.

Recently we have found
intriguing, but seemingly irregular configurations of a zoo of
topological vortices and antivortices in h
-
REMnO
3
. These configurations can be neatly
analyzed in terms of graph theory and this graph theoretical analysis

reflects the nature of
self
-
organized criticality in complexity phenomena as well as the condensation and eventual
annihilation processes of topological vortex
-
antivortex pairs.


Ferroelectric Tunnel Junctions:

Controlling Electron and Spin Transport by

Ferroelectric Polarization


Evgeny Tsymbal

Department of Physics and Astronomy, University of Nebraska
-
Lincoln

Tunnel junctions are useful electronic devices in which current
-
carrying electrons can
quantum
-
mechanically be transmitted between two metal ele
ctrodes across a very thin
insulating barrier layer. A particular example is a magnetic tunnel junction, where electrical
resistance depends on magnetization orientation of the two ferromagnetic electrodes


the
phenomenon known as tunneling magnetoresista
nce (TMR). So far, however, almost all the
existing tunnel junctions were based on non
-
polar dielectrics. An exciting possibility to extend
the functionality of tunnel junctions is to use a ferroelectric insulator as a barrier to create a
ferroelectric tun
nel junction (FTJ). [1] The key property of FTJ is tunneling electroresistance
(TER) that is the change in electrical resistance of a FTJ with reversal of ferroelectric
polarization. Functional properties of a FTJ can be further extended by ferromagnetic
e
lectrodes to create a multiferroic tunnel junction (MFTJ). In such a MFTJ the transport spin
polarization and TMR are affected by ferroelectric polarization of the barrier. [1] Thus, MFTJs
represent four
-
state resistance devices that can be controlled bot
h by electric and magnetic
fields. This talk will address the physics of FTJs and MFTJs based on recent modeling and
experiments.


[1]

E. Y. Tsymbal and H. Kohlstedt,
Science
313, 181 (2006).


Emergent Phenomena in Spatially Confined Manganites

Jian She
n

Department of Physics, Fudan University, Shanghai 200433


Colossal electroresistance, colossal magnetoresistance, high T
c

superconductivity and
the metal
-
insulator transition are some of the fascinating emergent behaviors found in complex
materials; howe
ver there is as yet no known model that is capable of fully explaining any one
of these behaviors let alone a unifying understanding capable of explaining the effects of
complexity on emergent behavior as a whole. One common trait that many of these compl
ex
materials share is electronic phase separation. For this reason, a fuller understanding of
electronic phase separation should have far reaching implications across a wide range of
materials.

We will discuss recent work on a novel spatial confinement
technique that has led to
some fascinating new discoveries on the role of electronic phase separation (EPS) in
manganites. In transport measurements on unconfined systems where device size is larger
than the inherent electronic phase domains, current bypa
sses regions of high resistance in favor
of regions with lower resistance, because the probing electrons will follow the path of least
resistance. By confining complex materials exhibiting EPS to length scales smaller than the
electronic phase domains tha
t reside within them, it is possible to simultaneously probe
multiple resistive regions. This method allows for a much more complete view of the phases
residing in a material and gives vital information on phase formation, movement and
fluctuation. Since

these phase separated regions also posses varied properties, this technique
promises to lead to unexpected functionalities for future device applications.

Reduced dimensionality studies on complex oxides sit at the intersection of
fundamental science an
d technological application. With the wide range of useful
materials

ferromagnets, antiferromagnets, high T
C

superconductors, multiferroics

present in
the complex oxides class, the burgeoning field of oxide electronics presents itself as the future
of dev
ice design. Not only will spatial confinement studies on correlated systems give us new
insights into fundamental physics, these studies are a vital step in the creation and
implementation of practical oxide electronic devices. Further, the formation an
d coexistence
of multiple electronic phases in a single system have been suggested as purely emergent
phenomena
4
; establishing a common language capable of discussing complexity in thes
e
material systems could find overlap with other fields, such as Biology or Economics, where
complex systems are the norm.

Neutron scattering study on crystalline and magnetic structure and excitations of the
Fe
-
based superconductors

Wei Bao

Department of
Physics, Renmin University of China, Beijing 100872


We will present our neutron scattering investigation on the recently discovered
Fe
-
based high transition temperature superconductor materials. Our neutron diffraction results
of magnetic order and lattic
e structure of
the 1111 [1], 122 [2] and 11 [3] families of the
Fe
-
based superconductor materials uncover a close relation between lattice distortion and
magnetic interaction. The magnetostructural transitions resemble those previously discovered
in either

vanandiumsesquioxide [4] or in manganites [5], which are naturally explained by an
orbital ordering transition. The prevailing theoretic explanation of the antiferromagnetic
transition in these parent materials, the spin
-
density
-
wave mechanism, however, i
s not
sufficient to explain our experimental results.

The antiferromagnetic phase can coexist with the superconducting phase in the 122
system [6], while a phase diagram more resembling that of cuprate superconductors exists for
the 11 system [7]. The symm
etry of superconducting order parameter has strong signature in
magnetic excitation spectrum, and we observed the telltale spin resonance mode of the s+/
-

symmetry and the accompanying spin gap in the superconducting state of the 11
superconductor [8]. The

normal state was shown to exhibit single
-
lobed incommensurate
excitation continuum of a typical itinerant antiferromagnet, in contrast to spin
-
wave cone of a
localized antiferromagnet [8,9], supporting a Fermi liquid description of the normal state.

More
recently a new family of alkali metal intercalated iron selenide superconductors
of Tc above 30 K
is

discovered. We will present the determination of the sample composition,
crystal structure and magnetic order using neutron diffraction technique. Contrary

to previous
belief, the materials are mostly charge balanced, instead of heavily electron doped, with
appropriate chemical formula as A
x
Fe
2
-
x/2
Se
2

and Fe at valance +2 [10]. In superconducting
samples the Fe vacancies order into an almost perfect pattern
in a five times larger unit cell
[11,12]
. For all the superconductors,
A=K, Rb, Cs, (Tl,K), and (Tl,Rb) and x~0.8, a large
moment block checkerboard antiferromagnetic order is found to coexist with superconductivity
[12,13]. These results demonstrate a ver
y different kind of superconductors from all previous
iron based high Tc superconductors, and a new different mechanism is expected.

[1] Y. Qiu, W. Bao, Q. Huang
et al.
, Phys. Rev. Lett.
101
,

257002

(2008).

[2] Q. Huang, Y. Qiu, W. Bao
et al.
, Phys. Rev. L
ett.
101
,

257003

(2008); M. Kofu, Y. Qiu, W.
Bao
et al.
, New J. Phys.
11
,

055001

(2009).

[3] W. Bao, Y. Qiu, Q. Huang
et al.
, Phys. Rev. Lett.
102,

247001

(2009).

[4] W. Bao
et al.
, Phys. Rev. Lett.
78
,

507

(1997).

[5] W. Bao
et al.
, Phys. Rev. Lett.
78
, 5
43

(1997).

[6] H. Chen
et al.
, EPL
85
, 17006 (2009).

[7] T.J. Liu et al., Nature Materials
9
, 718 (2010).

[8] Y. Qiu, W. Bao, Y. Zhao
et al.
, Phys. Rev. Lett.
103
,
067008 (2009).

[9] D.N. Argyriou et al., Phys. Rev. B
81
, 220503

(
R)

(
2010).

[10] W. Bao et

al., arXiv:1102.3674 (2011)

[11] P. Zavalij et al., arXiv:1101.4882 (2011)

[12] W. Bao et al., arXiv:1102.0830 (2011)

[13] F. Ye et al., arXiv:1102.2882 (2011)


The effects of spin
-
orbit coupling in low
-
dimensional magnetism and

spintronics

Ruqian Wu

Depa
rtment of Physics and Astronomy, University of California, Irvine, CA

92697


Spin
-
orbit coupling is essential for various physical phenomena, from

magnetic
anisotropy, magneto
-
optical effect, to quantum spin Hall effect.

To appropriately integrate
spin
-
orb
it coupling in modern density

functional studies allow predicting and explaining new
properties and

systems. I will discuss results of our recent studies for magnetic

anisotropy,
Kondo effect and quantum spin Hall effect in magnetic thin

films and other na
nostructures.


Spin Dynamics in Rare Earth Titanates R
2
Ti
2
O
7

(R = Dy/Tb, Nd) studied by ac
susceptibility

Hao Zeng

Department of Physics, University at Buffalo, the State University of New York


We present a serial of our recent studies on rare earth titan
ates R
2
Ti
2
O
7

(R=Y, lanthanide),
with particular emphasis on the spin relaxation process with ac susceptibility as the main probe.
We studied the ac hybrid spin system Dy
x
Tb
2
-
x
Ti
2
O
7
. The two compounds at the composition
boundaries are spin ice Dy
2
Ti
2
O
7

and
spin liquid Tb
2
Ti
2
O
7
,

which are the two representative
geometrically frustrated pyrochlores. In addition to the known Dy
3+

single
-
ion peak at
T
s

(T
s

peak), we identified a new freezing
-
like peak associated with Tb
3+

spins (T
*

peak). We propose
that T
*

peak
s correspond to the low
-
lying crystal field levels of Tb
3+

spins, and the crystal field
scheme evolves as the composition
x

changes, which explains the phase diagram of
T
*
(
x
). The
finding of the T
*

peak in Dy
x
Tb
2
-
x
Ti
2
O
7

demonstrates the rich dynamical magn
etic behavior of
such systems, and provides experimental foundations for future explorations of hybrid rare
earth spin systems. It is known that the structure of R
2
Ti
2
O
7
transforms into monoclinic as the
ionic radius of R increases. We have investigated Nd
2
Ti
2
O
7

in this category. We showed
anisotropic paramagnetism arising mainly from the crystal electric field, and separated the
exchange contribution
θ
ex

and the crystal field contribution

θ
CF

to the total Weiss temperature
θ
W
. The
θ
ex

orders of magnitude higher than the magnetic ordering temperature
T
order

suggests
strong spin frustration in the system, despite the absence of a pyrochlore structure. Mo
re
importantly, we found in Nd
2
Ti
2
O
7

a novel field
-
induced slow spin relaxation in the
paramagnetic state. We propose that the field
-
induced slow spin relaxation is associated with
the cooperative behavior of the correlated regions formed by partially pola
rized spins through
spin correlations. We believe that the cooperative behavior can exist not only in geometrically
frustrated systems, but also in many more paramagnets with strong spin correlations, while
their observable microscopic effects are closely
associated with the degree of frustration in the
system.


Shape
-
Controlled Metal Nanoparticles: Synthesis, Characterization and Applications

Jian
-
Guo Zheng

The Laboratory for Electron and X
-
ray instrumentation (LEXI)

California Institute for Telecommunica
tions and Information Technology (Calit2)

University of California,
Irvine, CA 92697
-
2800


It is well
-
known that when material size is reduced to nanometer scale, the physical and
chemical properties of nanoscale materials, such as luminescence, conductivi
ty, and catalytic
activity, will be significantly different from their bulk counterpart, which may be attributed to
quantum confinement effect and large ratio of surface area to volume. Besides size and
composition, shape is also an important factor to con
trol the properties of
nanoparticles
.
Colloid chemists have gained excellent control over particle size for several spherical metal
and semiconductor compositions. These colloidal spherical
nanoparticles

may be used as
probes for biological diagnostic appl
ications, LED materials, lasers, and Raman
spectroscopy

enhancing materials. However, it was a challenge to synthetically control particle
shape. Some significant progresses have been made in synthesizing large quantities of
anisotropic nanoparticles
-

non
-
spherical structures with shape
-
dependent properties
-

in high
yield in last decade. This talk is going to give a brief review about shape
-
controlled metal
nanoparticles
, including their
synthetic strategies,
characterization techniques and applications.


Recent progresses on the rare earth magnets made by rapid solidification process (RSP)

W. C. Chang

Physics Department, Chung Cheng University, Chia
-
Yi, Taiwan

E
-
mail: phywcc@ccu.edu.tw.


Since last two decades, rare earth permanent magnets (REPM) have bee
n widely used in
office automation, computers and mobile communications, EV, HEV, wine turbines, etc. The main
processes for making REPM include sintering and epoxy bonding. The former is capable of
making anisotropic high performance magnets, while the la
tter is suitable for making thin wall
and tiny magnets, even though the magnetic property is usually isotropic and inferior to the
magnets made by the former. Nevertheless, the process is simple, easy and cheap. The original
materials for bonded magnets ar
e normally produced by rapid solidification process (RSP),
especially for single phase nanocrystalline NdFeB ribbons. However, this process has also been
applied for making nanocomposite RFeB and some other permanent magnets. In this presentation,
I will i
ntroduce the recent progresses in our lab
,

on the permanent magnets made by RSP, the
materials include nanocomposites RFeB ribbons, nanocrystalline bulk RFeB magnets, Sm(Co,
M)
7

(M=refractory elements) ribbons and Fe
-
B/FePt nanocomposites.


Probing Ferroel
ectric Functionality Cell
-
by
-
Cell by Aberration
-
Corrected STEM

S. J. Pennycook
(1,2)*, H. J. Chang(1)**, D. N. Leonard(1), M. P. Oxley(1,2), J. He(2,1), S. T.
Pantelides(2,1) and A. Y. Borisevich(1)

(1) Materials Science and Technology Division, Oak Ridge N
ational
Laboratory
, Oak Ridge,
TN 37831
-
6071, USA

(2) Department of Physics and
Astronomy
, Vanderbilt University, Nashville, TN 37235, USA

* E
-
mail:
pennycooksj@ornl.gov

** now at Korea Institute of Science and
Technology, 39
-
1
Hawolgok
-
dong, Seongbuk
-
gu,

Seoul 136
-
791,Korea


The recent introduction of next
-
generation aberration correctors has propelled the
capabilities of the scanning transmission electron microscope (STEM) to a whole new level of
sensitivity. The resolution has increased to well below 1
Å, allowing atomic column positions
to be extracted with unprecedented precision, and making possible the mapping of ferroelectric
displacements on a unit cell by unit cell basis. Furthermore, the STEM allows multiple,
simultaneous images to be acquired, Z
-
contrast images, dominated by the heavy cation
columns can be obtained with either bright field images (sensitive to oxygen column positions)
or electron energy loss signals (allowing mapping of elemental composition, electronic
structure and dielectric p
roperties. Correlating these signals at atomic resolution across
interfaces enables new insights into screening mechanisms and the role of lattice constraints,
such as suppression of octahedral rotations.

As a case study several examples will be shown of
BiFeO
3
, mapping polarization
direct from a Z
-
contrast image [1], mapping octahedral rotations across interfaces with
electrodes, the observation of an induced dielectric anomaly, the influence of vacancy ordering
on polarization and the use of column shape

analysis to locate and identify domain walls [2].
Experimental results will be complemented by model theoretical studies [3].

[1] A. Y. Borisevich et al., Phys. Rev. Lett.
105
, 087204 (2010).

[2] A. Y. Borisevich et al., ACS Nano,
4
, 6071 (2010).

[3] J. H
e at al.,
Phys. Rev. Lett.
,
105
, 227203 (2010).


Solid Oxide Fuel Cells for Clean Energy Generation

Xingbo Liu

Mechanical & Aerospace Engineering Department, West Virginia University

Morgantown, WV 26506
-
6106


The first patent on fuel cells was filed in 18
38, and they have received huge attentions
in recent years due to their many advantages over other energy generation devices such as
internal combustion engines and turbine systems. A fuel cell is an electrochemical cell that
converts chemical energy from
a fuel into electric energy. Electricity is generated from the
reaction between a fuel supply and an oxidizing agent. It is made up of three segments which
are sandwiched together: the anode, the electrolyte, and the cathode. Two chemical reactions
occur a
t the interfaces of the three different segments. The net result of the two reactions is that
fuel is consumed, water or carbon dioxide is created, and an electric current is created, which
can be used to power electrical devices, normally referred to as t
he load. This talk will give an
overview of various types of fuel cells, with the focus on solid oxide fuel cell (SOFC). The
recent progress, as well as the challenges in both material and system levels in SOFC systems,
will be presented.


Irradiation Indu
ced Structural and Magnetic Property Changes in Nanoparticle
Granular Films

You Qiang

Physics Department, University of Idaho, Moscow ID 83844


Future generation IV nuclear reactors in US and storage medium in irradiation
environment are expected to meet t
he standards of enhanced safety and economical
compatibility. This research contributes in part to meet this objective through the understanding
of irradiation induced structural and magnetic property changes in nanomaterials. Fe
3
O
4

and
FeO+Fe
3
N granular f
ilms, created by the third generation nanocluster source were irradiated.
The pristine magnetite (Fe
3
O
4
) granular films with an average grain size of 3 nm are
superparamagnetic in nature. These films enriched ferromagnetic behavior under the irradiation
ex
posure of 5.5 MeV Si
2+

ions to a fluence of 10
16

ions/cm
2

at room temperature. After
irradiation, the average grain size and magnetic domain size of the films showed dramatic
increase. On the contrary, the magnetic properties of FeO+Fe
3
N granular films wer
e unaffected
even when irradiated with 2 MeV He
+

ions to a fluence of 3 × 10
15

ions/cm
2

at room
temperature. Surprisingly, the saturation magnetization (30 emu/g), coercivity (87.9 Oe),
magnetic remanence (3.2 emu/g) and magnetic susceptibility measurement
s of these granular
films remained unaltered even after irradiation. This is a fascinating behavior because this
meets some of the requirements for advanced data storage applications in extreme environment.
Electron beam exposure on iron nanoparticles is we
ll known to cause structural changes. The
mystery behind this behavior
-
particle size growth and oxide layer growth in core
-
shell iron
nanoparticles due to electron beam irradiation in TEM ultrahigh vacuum
-

was investigated in
this study.


Three
-
dimension
al dislocation dynamics in epitaxial thin films

Lizhi Sun

Departments of Civil & Environmental Engineering and Chemical Engineering & Materials
Science, University of California, Irvine, CA 92697
-
2175;
Email:
lsun@uci.edu


Dislocation dynamics in three
-
dimensional setting has been the subject of extensive
and great interest since 1990 due to the significant roles it plays in plastic deformation, work
hardening and softening, fracture mechanics and electronic device fabric
ation. However, a
long
-
standing problem inherent to the classical dislocation dynamics is that simulation results
heavily depend on segment sizes, which substantially reduces the reliability of simulation
results. We present a new three
-
dimensional disloca
tion dynamics model together with its
physical background. This new model fully incorporates the interactions among differential
dislocation segments. The proposed model is applied to simulate the effect of dislocations on
the mechanical performance of epi
taxial thin films. The interactions among the dislocation
loop, free surface and interface are rigorously computed by decomposing this complicated
problem into two relatively simple sub
-
problems. This model is allowed to determine the
critical thickness of

thin films for a surface loop to nucleate and to simulate how a surface loop
evolves into two threading dislocations. Furthermore, the relationship between the film
thickness and yield strength is constructed and compared with the Hall
-
Petch relation.


T
hermoelectric Nanocomposites and Microscale Modules

Jing
-
Feng Li

Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084


Thermoelectric

materials are
technically
important
for
energy harvesting and
conversion

technology, par
ticularly for recovering
an enormous amount of unused waste heat
produced by industrial processes and automotive exhaust.

Good thermoelectric materials must
have high Seebeck coefficient, good electrical conductivity and low thermal conductivity.
However,
it is very difficult to control the above three parameters independently, which often
counter each other. This talk will give two representative examples showing that thermoelectric
properties can be enhanced in nanocomposite
materials
. One example is AgPb
m
SbTe
m+2

system
bulk materials with
intrinsically

embedded nanoscale inclusions. Our work demonstrated that
a
high ZT value up to 1.5 at 700 K can be achieved in this material fabricated by a facile process
combining mechanical alloying (MA) and spark plas
ma sintering (SPS) methods. Another
example is an
extrinsic

approach by incorporating nanoparticles into a thermoelectric
compound matrix to form

composites
, but the key point is how to effectively reduce thermal
conductivity to a greater degree than elect
rical conductivity for ZT enhancement. O
ur recent
studies revealed that nano
-
SiC dispersed Bi
2
Te
3

polycrystalline materials show not only
enhanced thermoelectric performance but also better mechanical strength and fracture
toughness. By using such a kind o
f strengthened and fine
-
grained Bi
2
Te
3
-
based materials
prepared by spark plasma sintering (SPS), miniaturized thermoelectric (TE) modules were
fabricated by combining mechanical cutting and photolithograph processes. A microscale
thermoelectric module with

pillars that are as fine as 200 × 400 μm
2

in cross
-
section with a
height up to 600 μm was fabricated, whose maximum open output voltage was about 20 mV
when heated under a100 W lamp.


Crystallization of ferroelectric lead zirconate titanate thin films un
der microwave
irradiation


Zhan Jie Wang

Shenyang National Laboratory for Materials Science, Institute of Metal Research,


Chinese Academy of Sciences (CAS), 72 Wenhua Road, Shenyang 110016

E
-
mail:
wangzj@imr.ac.cn


Lead zirconate titanate
(Pb(Zr
x
Ti
1
-
x
)O
3
: PZT)

have excellent ferroelectric, pyroelectric
and piezoelectric properties.
Techniques for the deposition of PZT thin films have been
developed rapidly in recent years because of the large number of potential a
pplications of PZT
thin films in nonvolatile ferroelectric random
-
access memories (FeRAMs) and
micro
-
electromechanical systems (MEMS) such as micro
-
scanning mirror devices and atomic
force microscopy (AFM) cantilevers
.

It is imperative to decrease the ther
mal processing
temperature and time to prevent interdiffusion between the elements of the films and the
substrate and to prevent the evaporation of lead and lead oxide from the surface of the films. In
this study, PZT thin films were coated on Pt/Ti/SiO
2
/Si substrates by a sol
-
gel method and then
crystallized by multimode 28 GHz microwave irradiation or single
-
mode 2.45 GHz microwave
irradiation in the magnetic field. The crystalline phases and microstructures as well as the
electrical properties of the m
icrowave
-
irradiated PZT films were investigated. Experimental
results indicated that microwave irradiation is effective for obtaining well
-
crystallized PZT thin
films with good electric properties at low temperatures or in a short time.

1

Z. J. Wang, H. Ko
kawa, H. Takizawa, M. Ichiki and R. Maeda,
Appl. Phys. Lett.

86
, 212903
(2005).

2

Z. J. Wang, Z. P. Cao, Y. Otsuka,
N. Yoshikawa

and H. Kokawa,
Appl. Phys. Lett.

92
, 222905
(2008).

3

Z. P. Cao, Z. J. Wang, N. Yoshikawa and S. Taniguchi,
J.

Phys.D: Appl. Ph
ys
.

41
, 092003
(2008).

4

Z. J. Wang, Y. Otsuka, Z. P. Cao,
N. Yoshikawa and
H. Kokawa,

Jpn. J. Appl. Phys.

47
, 7519
(2008).

5

Z. J. Wang, Y. Otsuka, Z. P. Cao
and
H. Kokawa,

Jpn. J. Appl. Phys.

48
,
09KA01 (2009).

6
Z. J. Wang,
Y. N. Chen, Y. Otsuka, M. W.
Zhu and H. Kokawa
,
J. Am.
Ceram. Soc.

94
, 404

(2011).