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UNIVERSITY OF OULU P .O. Box 8000 F I -90014 UNIVERSITY OF OULU FINLAND
A C T A U N I V E R S I T A T I S O U L U E N S I S
Professor Esa Hohtola
University Lecturer Santeri Palviainen
Postdoctoral research fellow Sanna Taskila
Professor Olli Vuolteenaho
University Lecturer Veli-Matti Ulvinen
Director Sinikka Eskelinen
Professor Jari Juga
University Lecturer Anu Soikkeli
Professor Olli Vuolteenaho
Publications Editor Kirsti Nurkkala
ISBN 978-952-62-1359-0 (Paperback)ISBN 978-952-62-1360-6 (PDF)ISSN 0355-3213 (Print)ISSN 1796-2226 (Online)
U N I V E R S I TAT I S O U L U E N S I SACTAC
TECHNICA
U N I V E R S I TAT I S O U L U E N S I SACTAC
TECHNICA
OULU 2016
C 584
Hanna Kähäri
A ROOM-TEMPERATURE FABRICATION METHOD FOR MICROWAVE DIELECTRIC Li2MoO4 CERAMICS AND THEIR APPLICABILITY FOR ANTENNAS
UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU;FACULTY OF INFORMATION TECHNOLOGY AND ELECTRICAL ENGINEERING
C 584
ACTA
Hanna K
ähäri
C584etukansi.kesken.fm Page 1 Friday, September 16, 2016 10:35 AM
A C T A U N I V E R S I T A T I S O U L U E N S I SC Te c h n i c a 5 8 4
HANNA KÄHÄRI
A ROOM-TEMPERATURE FABRICATION METHOD FOR MICROWAVE DIELECTRIC Li2MoO4 CERAMICS AND THEIR APPLICABILITY FOR ANTENNAS
Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in the OP auditorium (L10), Linnanmaa, on 4November 2016, at 12 noon
UNIVERSITY OF OULU, OULU 2016
Copyright © 2016Acta Univ. Oul. C 584, 2016
Supervised byProfessor Heli JantunenDocent Jari Juuti
Reviewed byProfessor Robert FreerProfessor Eung Soo Kim
ISBN 978-952-62-1359-0 (Paperback)ISBN 978-952-62-1360-6 (PDF)
ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)
Cover DesignRaimo Ahonen
JUVENES PRINTTAMPERE 2016
OpponentsProfessor Robert FreerProfessor Maarit Karppinen
Kähäri, Hanna, A room-temperature fabrication method for microwave dielectricLi2MoO4 ceramics and their applicability for antennas. University of Oulu Graduate School; University of Oulu, Faculty of Information Technologyand Electrical EngineeringActa Univ. Oul. C 584, 2016University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland
Abstract
This work presents a method for the fabrication of Li2MoO4 ceramics at room-temperature basedon utilizing a small amount of water with Li2MoO4 powder. The densification of the ceramic takesplace during pressing. Thus the shape and size of the final ceramic compact can easily be managedby controlling the mould dimensions and the amount of material. Post-processing at 120 °C isapplied to remove residual water from the compact. This post-processing temperature can bechosen to be suitable to the other materials integrated, such as the substrate or electrodes, as longas the post-processing time is adequate to remove the residual water. The dielectric properties(relative permittivity of 5.1 and a loss tangent value of 0.00035 at 9.6 GHz) after optimization ofthe powder particle size, sample pressing pressure, and post-processing time were similar to thoseachieved for Li2MoO4 ceramics fabricated by sintering at 540 °C.
The dielectric properties of Li2MoO4 ceramics were also modified using composite methods.For example, an addition of 10 volume-% of BaTiO3 increased the relative permittivity from 6.4to 9.7 and the loss tangent value from 0.0006 to 0.011 at 1 GHz. To investigate the thermaldependence of the permittivity, different amounts of rutile TiO2 were incorporated into a Li2MoO4ceramic matrix fabricated with the method described in this work. As the amount of TiO2increased from 10 to 30 volume-%, the thermal coefficient of permittivity decreased from 180ppm/°C to -170 ppm/°C. The low processing temperature made the fabrication approachintroduced here feasible for silver electrode integration without the formation of extra phases,which were observed in sintered samples with similar compositions in another study.
A patch antenna was realized utilizing a Li2MoO4 ceramic disk fabricated by the room-temperature method. The antenna operating at ~4 GHz showed reasonably good performance. Arelative humidity of 80% lowered the resonant frequency by 3.25% from the initial value, andreduced the total and radiation efficiencies of the antenna by ~2 dB. The changes were slowlyreversible. Use of a silicone conformal coating reduced the shift of the resonant frequency to1.26% from the initial value and also reduced the effect on efficiencies to ~1 dB. The use of thecoating also speeded up the reversibility of the changes when the humidity was decreased.
Keywords: antenna, densification, dielectric, lithium molybdate, loss tangent,permittivity
Kähäri, Hanna, Menetelmä mikroaaltoalueen dielektristen Li2MoO4-keraamienvalmistukseen huoneenlämmössä ja niiden soveltuvuus antenneihin. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Tieto- ja sähkötekniikan tiedekuntaActa Univ. Oul. C 584, 2016Oulun yliopisto, PL 8000, 90014 Oulun yliopisto
Tiivistelmä
Tässä työssä esitellään menetelmä, jolla Li2MoO4-keraameja voidaan valmistaa huoneenlämpö-tilassa. Menetelmä hyödyntää pientä määrää Li2MoO4-vesiliuosta ja sen kiteytymistä. Keraamitiivistyy kappaletta puristettaessa, joten sen koko ja muoto ovat sama kuin muotilla riippuenvain keraamin määrästä. Kappaleeseen jäänyt vesi poistetaan lämpökäsittelyllä yleensä 120 cel-siusasteessa. Jälkikäsittelylämpötila voidaan valita muiden integroitavien materiaalien mukaan,kuten alusta- tai elektrodimateriaalin, kunhan jälkikäsittelyaikaa muokataan vastaavasti, jottakaikki vesi poistuu. Optimoimalla Li2MoO4-jauheen partikkelikokoa, puristuspainetta ja jälkikä-sittelyaikaa saavutettiin samankaltaiset dielektriset ominaisuudet taajuudella 9,6 GHz (suhteelli-nen permittiivisyys 5,1 ja häviötangentti 0,00035) kuin Li2MoO4-keraameilla, jotka on sintrattu540 celsiusasteessa.
Li2MoO4-keraamien dielektrisiä ominaisuuksia muokattiin myös lisäaineilla. Esimerkiksi 10tilavuus-% BaTiO3-jauhetta kasvatti suhteellista permittiivisyyttä taajuudella 1 GHz arvosta 6,4arvoon 9,7 ja häviötangenttia arvosta 0,0006 arvoon 0,011. Myös eri määriä TiO2-jauhetta (rutii-li) lisättiin Li2MoO4-matriisiin permittiivisyyden lämpötilariippuvuuden tutkimiseksi. TiO2-jau-heen määrän kasvaessa 10 tilavuusprosentista 30 tilavuusprosenttiin laski permittiivisyyden läm-pötilariippuvuus arvosta 180 ppm/°C arvoon -170 ppm/°C. Matalan käsittelylämpötilan ansiostatyössä esitelty valmistusmenetelmä soveltui käytettäväksi hopeaelektrodien kanssa. Aiemmantutkimuksen mukaan nämä komposiittimateriaalit muodostivat ei-toivottuja faaseja sintrattaessahopean kanssa.
Menetelmällä valmistettua Li2MoO4-keraamikiekkoa käytettiin mikroliuska-antennin val-mistuksessa. Taajuudella 4 GHz toimivan antennin suorituskyky oli suunnitellun kaltainen. 80prosentin suhteellinen ilmankosteus laski resonanssitaajuutta 3,25 % alkuperäisestä arvosta javähensi antennin kokonais- ja säteilytehokkuutta noin 2 dB. Muutokset palautuivat hitaasti. Sili-konisuojalakan käyttö vähensi taajuuden laskua 1,26 prosenttiin alkuperäisestä arvosta ja tehok-kuudet laskivat vain noin 1 dB. Suojalakan käyttö nopeutti muutosten palautuvuutta ilmankos-teuden laskiessa.
Asiasanat: antennit, eristeet, häviötangentti, litiummolybdaatti, permittiivisyys,tiivistyminen
To my grandpa, Samuli Putaansuu (1925–2016). The greatest wisdom rests in the heart.
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Acknowledgements
This work was done as a part of the European Research Council funded projects
Ultimate Ceramics and LTCeramics at the Microelectronics Research Unit of the
University of Oulu. Financial support by the Riitta and Jorma J. Takanen
Foundation, the Seppo Säynäjäkangas Foundation, the Tauno Tönning Foundation,
the Ulla Tuominen Foundation, the KAUTE Foundation, the Finnish Foundation
for Technology Promotion, and the Foundation for Women in Academic Research
is acknowledged.
I am deeply grateful to my Principal supervisor, Prof. Heli Jantunen, for giving
me the chance to put my wild ideas into practice, and to Co-supervisor, Docent Jari
Juuti, for his keen eye on details. I also express my gratitude to Prof. Arthur Hill
for his insightful comments and language editing of this thesis as well as the
original papers. I also acknowledge my co-authors Dr. Merja Teirikangas and Lic.
Tech. Prasadh Ramachandran for their work.
My friends and colleagues in the Microelectronics Research Unit have been of
a tremendous help in the professional and not-so-professional challenges during the
work on this thesis. Thank you all! In addition, I am thankful for the kind assistance
of the personnel in the Center of Microscopy and Nanotechnology.
I thank my friends for exceptionally bad movie-nights, great dinners, and good
conversations reminding me that there actually is more to life than work. I want to
thank my mother for her ever continuing support, my father for his solicitude, and
my sister and brother for being there for me through thick and thin. I am grateful
to have such a warm and encouraging expanded family. Finally, I want to thank my
beloved Jere (and Sini) for giving me strength (literally!), love, and acceptance.
Oulu, July 2016 Hanna Kähäri
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List of abbreviations and symbols
αL Linear expansion coefficient
εr Relative permittivity
Ag Silver
BaTiO3 Barium titanate
CTE Coefficient of thermal expansion
f0 Peak resonance frequency
FESEM Field emission scanning electron microscope
GaAs Gallium arsenide
GPS Global positioning system
Li2CO3 Lithium carbonate
Li2MoO4 Lithium molybdate
LiOH Lithium hydroxide
LTCC Low-temperature co-fired ceramics
MoO3 Molybdenum oxide
RH Relative humidity
tan δ Dielectric loss tangent
TCεr Temperature coefficient of permittivity
TCF Temperature coefficient of resonant frequency
TiO2 Titanium oxide
Ts Sintering temperature
ULTCC Ultra-low temperature co-fired ceramics
WLAN Wireless local area network
XRD X-ray diffraction
ZrO2 Zirconium oxide
Q Quality factor
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List of original papers
This thesis is based on the following four publications, which are referred to
throughout the text by their Roman numerals:
I Kähäri H, Teirikangas M, Juuti J & Jantunen H (2014) Dielectric properties of lithium molybdate ceramic fabricated at room temperature. Journal of the American Ceramic Society 97(11): 3378–3379.
II Kähäri H, Teirikangas M, Juuti J & Jantunen H (2015) Improvements and modifications to room-temperature fabrication method for dielectric Li2MoO4 ceramics. Journal of the American Ceramic Society 98(3): 687–689.
III Kähäri H, Teirikangas M, Juuti J & Jantunen H (2016) Room-temperature fabrication of microwave dielectric Li2MoO4–TiO2 composite ceramics. Ceramics International 64(1): 71–75.
IV Kähäri H, Ramachandran P, Juuti J & Jantunen H (2016) Room-temperature densified Li2MoO4 ceramic patch antenna and the effect of humidity. Manuscript.
Paper I presented a room-temperature fabrication method for lithium molybdate
(Li2MoO4) ceramic and its microwave dielectric properties. In Paper II different
parameters of the densification method were optimized and the modification of
dielectric properties with metal oxides was introduced. The applicability of the
method in fabrication of composites was studied in Paper III. Paper IV presented
the first application, a patch antenna, realized with a Li2MoO4 ceramic disk
fabricated by the room-temperature method and the use of a conformal coating.
In papers I–III the ideas for the fabrication method, ceramic samples, the
crystal structure analysis, and the microstructure imaging were done by the author.
In papers I and II the dielectric properties were measured at high-frequency by a
co-author. In paper II the dielectric measurements at 1 GHz were done by the author.
In papers III–IV all the measurements were done by the author. The antenna in
Paper IV was designed and simulated by a co-author and fabricated by the author.
The experimental results were discussed together with the co-authors. Each original
paper was written by the author with the contribution of the co-authors.
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Contents
Abstract
Tiivistelmä
Acknowledgements 9 List of abbreviations and symbols 11 List of original papers 13 Contents 15 1 Introduction 17
1.1 Objective and outline of the thesis .......................................................... 19 2 Microwave dielectric ceramics 21
2.1 Key properties of microwave dielectric ceramics ................................... 21 2.1.1 Relative permittivity ..................................................................... 21 2.1.2 Dielectric loss ............................................................................... 22 2.1.3 Temperature coefficient of permittivity ........................................ 22 2.1.4 Chemical compatibility with other materials ................................ 23
2.2 Engineering of dielectric properties ........................................................ 23 2.3 Sintering of ceramics .............................................................................. 24
3 Materials and methods 27 3.1 Lithium molybdate (Li2MoO4) ................................................................ 27 3.2 Rutile TiO2 and BaTiO3 ........................................................................... 28 3.3 Characterization of the fabricated ceramics ............................................ 28
4 Room-temperature fabrication method 31 4.1 The effect of pressure .............................................................................. 33 4.2 The effect of the amount of water ........................................................... 34 4.3 The effect of post-processing temperature .............................................. 34 4.4 The effect of powder particle size ........................................................... 35 4.5 The effect of insoluble additives ............................................................. 35
5 The applicability of room-temperature fabricated Li2MoO4
ceramics for antennas 41 5.1 The effect of humidity ............................................................................. 44 5.2 The effect of temperature ........................................................................ 48
6 Summary and conclusions 51 References 53 Original papers 57
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1 Introduction
Devices utilizing the microwave frequency range (300 MHz – 300 GHz) have
become an essential part of our everyday life. Weather satellites give us warning of
forthcoming nimbuses, base stations enable us to communicate in real time with
our families, friends and colleagues, and direct broadcast satellites transmit
television picture from the centre of events to all parts of the world. The most
common devices utilizing the microwave frequency range are, however, portable
devices, such as smart phones, tablets, and different wearable gadgets with wireless
communication technologies ranging from Bluetooth and wireless local area
networks (WLAN) to mobile broadband technologies like 4G and the global
positioning system (GPS). In order to be able to miniaturize these devices to
handheld sizes, materials with excellent electrical/dielectric performance are
necessary.
One advantageous material group which enables such miniaturization is
microwave dielectric ceramics. They can be used for the miniaturization of
antennas, because, compared to air, they exhibit a high permittivity which decreases
the wavelength of microwaves by a factor approximately equal to the square root
of the permittivity (Fig. 1). [1] One approach for the miniaturization is the use of
microwave dielectric ceramics in Low-Temperature Co-fired Ceramics technology
(LTCC). In LTCC technology thin sheets of mainly ceramic material with printed
conductors and suitable functional inks are stacked together, laminated, and
sintered to produce a multilayer package with integrated components, such as
resistors, capacitors, inductors, filters, and also antennas. [2]
18
Fig. 1. Illustration of the ideal effect of relative permittivity (εr) on the wavelength of a
microwave.
One main challenge with the microwave dielectric ceramics is the high sintering
temperature which is required in order for the ceramics to achieve their final
optimal properties through densification. Generally, the dielectric ceramics are
sintered at temperatures much higher than 1000 °C, or in the case of LTCC
technology, at about 900 °C [2]. In addition to high energy consumption, such high
sintering temperatures cause many problems. The reactivity with other materials
increases, which can cause unwanted extra-phases with unexpected properties, and
volatile components can evaporate affecting the composition and therefore the
properties of the ceramic. The high processing temperature also complicates
integration with other materials, such as low melting temperature electrode metals,
semiconductors like silicon (Si) and gallium arsenide (GaAs), or polymers.
Furthermore, during the sintering process the formed compact shrinks, making the
dimensional management of the final product demanding. Thus much effort has
been expended in attempts to decrease the sintering temperature (Ts) of microwave
dielectric ceramics without losing their feasibility for practical applications. [3]
εr = 1 (vacuum)
εr = 9 εr = 25
19
Ceramic materials with Ts < 700 °C are called ultra-low temperature firing ceramics.
The possibility to use them in Ultra-Low Temperature Co-fired Ceramic
technology (ULTCC), similar to LTCC, has generated much research lately. The
materials presented in scientific publications can be divided into two main groups:
glasses and glass-ceramics, utilizing a low softening temperature glass with a
ceramic powder, and oxide compositions with an intrinsic ultra-low sintering
temperature, which have been derived from vanadates, tungstates, tellurates,
borates, and molybdates. [3] One ULTCC material candidate proposed is lithium
molybdate (Li2MoO4), which has been sintered at a temperature of 540 °C [4].
Some other microwave dielectric materials with even lower sintering temperatures
have also been found from the group of molybdates. These include Li2Mo4O13 (Ts
= 520 °C) [4], K2Mo3O10 (Ts = 520 °C) [5], K2Mo2O7 (Ts = 460 °C) [5], NaAgMoO4
(Ts = 400 °C) [6], Na6Mo11O36 (Ts = 510 °C) [7], and Ag2MoO4 (Ts = 450 °C) [8].
However, these temperatures are still too high for integration with, for example,
commonly used polymer substrates. Therefore, new openings in the field of
densification of ceramics at even lower temperatures or a new process approach
would be very valuable.
1.1 Objective and outline of the thesis
This work presents a new fabrication method for Li2MoO4 ceramics. Chapter 2
consists of an overview of the key properties of microwave dielectric ceramics, the
conventional fabrication method of ceramic components, and composite
technology. Liquid phase sintering is discussed in more detail because of the
similarities with the method in this work. The materials and characterization
methods used in this work are described in Chapter 3. Chapter 4 introduces the new
fabrication method for Li2MoO4 ceramics in detail and the effects of different
fabrication parameters are discussed based on Papers I–III. A patch antenna realized
with a Li2MoO4 ceramic component fabricated by the new method and the effects
of humidity on the antenna parameters both with and without a moisture protective
conformal coating are investigated in Chapter 5 (Paper IV). This chapter also
introduces the thermal stabilization of permittivity studied in Paper III. Chapter 6
finalizes the work by summarizing the results and discussing the challenges and
future possibilities of the fabrication method.
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2 Microwave dielectric ceramics
Microwave dielectric ceramics belong to the class of electroceramics, which can
be defined as solid materials composed mainly of inorganic non-metallic materials
having electrical performance [9, 10]. A large number of microwave dielectric
ceramics are known with varying properties, providing suitable materials for
miniaturized components and large area applications alike [2, 3]. Furthermore, the
properties can be engineered to optimize the critical dielectric parameters [11].
Conventionally ceramics are fabricated from powders, which are formulated
and compacted with suitable additives under pressure to a desired shape, and then
consolidated by sintering. In the sintering process solid particles in contact adhere
and grow together into a solid body. Usually sintering is also used to reduce the
porosity of a ceramic compact, leading to densification and size reduction of the
compact. [12, 13]
2.1 Key properties of microwave dielectric ceramics
Key properties of microwave dielectric ceramics required for practical applications
are suitable relative permittivity, low dielectric loss, temperature stable dielectric
properties, and chemical compatibility with the electrode metal. These are
described in this chapter in more detail. Other important characteristics include
densification behaviour and coefficient of thermal expansion (CTE) which are
matched with other materials in the integrations, a high thermal conductivity, and
good mechanical properties. [3, 14]
2.1.1 Relative permittivity
The relative permittivity (εr) of a material indicates its capacity to store energy
when an electric field is applied to it. It is compared with the energy storing capacity
of a vacuum, which has a relative permittivity of 1 by definition. This capacity
results from a dipole moment that the material acquires, known as polarization.
Polarization can occur in all materials by way of a small displacement of the
electrons in an atom relative to the nucleus. In ionic materials there is also a relative
displacement of anion and cation sub-lattices. In dipolar materials the applied
electric field can orient the molecules as well. In addition, there is a limited
transport of charge carriers until they are stopped at a potential barrier, such as a
grain boundary or a phase boundary. This is called space charge polarization. At
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microwave frequencies the permittivity derives predominantly from electronic and
ionic polarization mechanisms. The frequency dependence of permittivity is related
to the inertia of the charges, meaning that the polarization does not occur
instantaneously with the application of an electric field. [1, 10]
2.1.2 Dielectric loss
The dielectric loss tangent (tan δ) of a material expresses the quantity of the
electrical energy which is dissipated as heat [10]. The origin of dielectric losses can
be attributed to the different physical properties, such as electrical conduction, but
it can also be considered to be related to the delay between the change in electric
field and the polarization vectors. Dielectric loss of a ceramic is composed of
intrinsic and extrinsic losses. Intrinsic dielectric losses are related to the losses in a
perfect crystal and they depend on the crystal symmetry, the frequency of the
changing electric field and the temperature. Extrinsic losses are caused by
imperfections in the structure such as impurities, microstructural defects, porosity,
micro-cracks, grain boundaries, and so on. [1]
The reciprocal of the tan δ value is known as the quality factor (Q). It has for
long been considered that since the dielectric loss is proportional to frequency, the
product of Q and frequency, Q x f, is approximately constant. However, a
significant increase in the Q x f value with frequency has been observed for some
typical microwave dielectric ceramics [15]. This frequency dependence is
attributed to the extrinsic dielectric loss, which is unavoidable in actual microwave
dielectric ceramics. Therefore, the measurement frequency should always be noted
with the Q x f value.
2.1.3 Temperature coefficient of permittivity
The temperature coefficient of permittivity (TCεr) indicates how much the
permittivity of a material changes with temperature. This variation is very
important for practical applications because a common operation temperature can
easily range from -30 to +80 °C. [2] TCεr is expressed as parts per million per
degree Celsius (ppm/°C). The temperature stability of a microwave dielectric
ceramic can also be expressed by a temperature coefficient of resonant frequency
(TCF). TCF is related to the linear expansion coefficient (αL) and TCεr by equation
(1). [1]
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= −(0.5 + ) (1)
2.1.4 Chemical compatibility with other materials
The ceramic material used in an application should not react unexpectedly with
other integrated materials, because formation of additional phases may degrade the
dielectric properties. Typically, the chemical compatibility is studied at least with
the electrode metal at the sintering temperature. The final situation is, however,
more complicated since the functional patterns are typically fabricated using pastes.
The additives of the paste may change the reactivity of the system, so the chemical
compatibility with all materials in the ceramic component should be taken into
account. [2]
2.2 Engineering of dielectric properties
There are various strategies to improve the dielectric properties of ceramics.
Microstructural engineering may be implemented either by adjusting the
processing conditions or by modifying the powder formulation, particle size and
size distribution, etc. These affect the sintered microstructure at four levels: the
grain level, the grain boundary, the sub-grain, and the lattice level. Engineering at
the lattice level affects the Q value. Slowing the cooling rate or increasing the
annealing time often increases the Q value because of the ordering effects. Higher
Q values can also be related to preventing the formation of oxygen vacancies with
dopants. At the grain boundary level, the amount of impurities is typically
minimized to prevent defect formation inside the grains. Engineering at the grain
level is achieved by alterations to the composition by substitution, evaporation of
volatile materials, or by combining two different dielectric ceramics. [11] The
methods for the engineering of dielectric properties are, however, heavily related
to the ceramic in question and should be investigated on a case-specific basis.
Various dielectric mixing equations have been used to calculate estimations of
the values of εr, Q x f, and TCF of a two component microwave dielectric ceramic
system, also known as a two-phase ceramic composite [16–19]. One of them is the
Lichtenecker logarithmic mixing rule
= + (2)
where y1 and y2 refer to the volume fractions of materials 1 and 2, and ε1 and ε2
define the permittivities of the materials, respectively. The Lichtenecker mixing
24
rule has been found accurate for prediction of the relative permittivity of several
two-phase dielectric ceramic composite systems [18–20].
2.3 Sintering of ceramics
The sintering process requires a method and an energy source for material transport.
Material transport can take place by viscous flow, plastic flow, evaporation-
condensation, surface diffusion, and volume diffusion. [12] Different sintering
mechanisms are related to different mechanisms of material transport. For example,
in the case of viscous flow the sintering mechanism is called liquid phase sintering
and in the case of diffusion it is called solid state sintering. Typically, in the
sintering process of ceramics the densification of the material occurs by diffusion
or viscous flow, or their combination, for which heat provides the energy source.
The driving force is a decrease in the surface free energy of powdered compacts,
by replacing solid-vapour interfaces with solid-solid interfaces, in which case the
free-energy decrease is brought about by a decrease in surface area. [13]
In liquid phase sintering, a viscous fluid is formed at the sintering temperature.
When this liquid is formed, it tends to completely coat the solid particles thereby
eliminating solid-vapour interfaces. Hence pores become located in the liquid
phase. In each pore there is a negative pressure called capillary pressure. Capillary
pressure will tend to rearrange the solid particles in such a way as to give maximum
packing and a minimum of resultant pore surface. Initially this can take place by
particles sliding over one another. Subsequently, bridges are built up which will
collapse by the solution of small amounts of material at the contact points. When
this occurs, substantial rearrangements of neighbouring particles can take place
with an ultimate arrangement of solid particles consistent with high density. [21]
Once the rearrangement process has been completed, densely packed solid
particles separated by thin liquid films carry the major part of the compressive
stress at the contact points. The solubility at the contact points between particles is
larger than the solubility of other solid surfaces and this results in the transfer of
material away from the contact points allowing the centre-to-centre distance
between particles to be decreased and densification to take place. [21]
During the sintering process a certain number of grains will be oriented such
that the liquid will not penetrate completely between them. In this case, along a line
between the grains centres the material will all be solid and consequently, the rapid
densification corresponding to the liquid phase processes stops. [21]
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The rate of the liquid phase sintering depends strongly on temperature and the
amount and properties of the liquid phase [13, 21, 22]. If the volume percent of the
liquid at the sintering temperature is equal to about 35 volume percent or more,
complete densification can take place by the rearrangement process and no other
sintering process is required. For smaller liquid contents the major part of
densification must take place by solution and precipitation. [21, 22] Higher
temperatures result in an increase in the amount of liquid phase but a small particle
size of the powder is also advantageous since it provides a higher surface energy.
For larger particles higher pressure during compacting can introduce sufficient
energy to increase the sintering rate. [12]
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3 Materials and methods
3.1 Lithium molybdate (Li2MoO4)
Phenacite type structured Li2MoO4 consists of a three-dimensional network of
corner-linked, slightly distorted LiO4 and fairly regular MoO4 tetrahedra and it
belongs to the space group R3̅ [23]. Moser et al. have assumed incongruent melting
for Li2MoO4 at 698 °C according to DSC studies [24]. Melting points of 701±2 °C
[25] and 705 °C [26] have also been reported. Conventionally Li2MoO4 is prepared
by solid state synthesis and sintering at temperatures from 500 °C to about 620 °C
using Li2CO3 and MoO3 [4, 27, 28]. An aqueous solution method has also been
described which utilizes (NH4)6Mo7O24 and LiOH as starting materials [29].
Another exceptional method of preparing crystalline Li2MoO4 is by grinding
LiOH·H2O with MoO3 in air at room temperature for 10 minutes. This reaction
which produces anhydrous crystalline Li2MoO4 is driven by the liberation of water
from the crystalline lattice. [30] Li2MoO4 crystallizes in anhydrous form from
aqueous solution [31] which explains the formation of anhydrous crystals even
when fabricated in the presence of water.
In industry Li2MoO4 has typically been used for corrosion inhibition in air
conditioning and dehumidification devices [32]. Recently, Li2MoO4 crystals have
been found applicable for cryogenic phonon-scintillation detectors, which are used
to investigate some rare nuclear processes [33]. Microwave dielectric properties of
Li2MoO4 sintered at the ultra-low sintering temperature of 540 °C have been
reported by Zhou et al. [4, 34]. Samples with a density of 2.895 g/cm3 (95.5% of
the theoretical density of 3.03 g/cm3) exhibited εr ~5.5 and a tan δ value of 2.83 x
10-4 at a frequency of 13.051 GHz, and a large negative TCF of -160 ppm/°C.
According to the X-ray diffraction analysis of co-fired ceramics, there was no
observed reaction with either silver or aluminum powders. [4]
In this work the Li2MoO4 powder (99+%, Alfa Aesar, Karlsruhe, Germany)
particle size was reduced either by grinding with a mortar and pestle or by milling
in ethanol with ZrO2 milling media. The resulting powders were sieved with mesh
sizes of 180 µm, 90 µm, and/or 45 µm. (Papers I–IV) According to a laser
diffraction particle size distribution measurement (LS 13 320, Beckman Coulter,
USA), in the original commercial powder at least 95 % of the particles were smaller
than 500 µm, and at least 95 % of the particles of the sieved powders were smaller
than the chosen mesh size.
28
3.2 Rutile TiO2 and BaTiO3
Titanium dioxide (TiO2) in rutile phase has a tetragonal structure and it requires
sintering temperatures up to 1500 °C to attain dense samples. The relative
permittivity of ~100 and a low tan δ value of ~0.0001 at microwave frequencies
suggest that rutile TiO2 might be a suitable material for dielectric resonators.
However, the temperature stability of rutile is poor with an exceptionally high TCF
of ~450 ppm/°C. [35, 36] Therefore it has been widely used to compensate the
negative TCF values of other microwave dielectric ceramics [19, 37, 38].
Barium titanate (BaTiO3) is used in lead-free piezoelectric devices and
multilayer ceramic capacitors [39]. Depending on the particle size, it may be
sintered at temperatures from 1100 °C to above 1300 °C [40, 41]. BaTiO3 has been
used to fabricate high permittivity glass ceramic compositions with ultra-low
sintering temperatures [41]. It has been found that the microwave properties of
BaTiO3 ceramics are strongly affected by the grain and particle size [39, 40].
In this work TiO2 (rutile, 99.8%, Alfa Aesar, Karlsruhe, Germany) and BaTiO3
(>99.7%, Alfa Aesar, Karlsruhe, Germany) were used to increase the permittivity
of Li2MoO4 ceramics (Paper II). Rutile TiO2 was also used to investigate the
temperature stabilization in the case of room-temperature fabrication of Li2MoO4
(Paper III).
3.3 Characterization of the fabricated ceramics
Prior to the characterization, the fabricated Li2MoO4 samples were thinned and
polished with a P1200 carborundum paper (EcoWet, KWH Mirka Ltd, Jeppo,
Finland) using ethanol. The density of the disks was calculated from the dimensions
measured by a micrometer screw (Mitutoyo Co., Japan) and the weight measured
by a precision scale (XT620M, Precisa, Switzerland). The crystal structure of the
samples was investigated by X-ray diffraction (XRD, Bruker D8 Discover X-Ray
Diffractometer, Karlsruhe, Germany) using CuKα radiation and a step size of 0.01°.
The microstructure of the polished disks was studied by a field emission scanning
electron microscope (FESEM) (Zeiss Sigma, Carl Zeiss, Germany) equipped with
an energy-dispersive spectrometer for chemical composition examination. For
microstructure images the samples were polished at first with water-free diamond
solution and then coated with a thin layer of carbon.
At 1 GHz, the relative permittivity and loss tangent were measured with an RF
impedance/material analyser (E4991A, Agilent Technologies, Santa Clara, CA,
29
USA). Silver electrodes were sputtered on both sides of the samples for the
measurements. At higher frequencies the dielectric properties were measured with
a resonant mode technique using a Split Post Dielectric Resonator (QWED, Warsaw,
Poland) with a nominal resonant frequency of 9.97 GHz. To determine the
temperature coefficients of permittivity (TCεr), measurements were performed
every 10 degrees between temperatures of 25 °C and 85 °C.
Reflection loss (S11) measurements for antennas, realized by utilizing room-
temperature fabricated Li2MoO4 ceramic components, were conducted with a
network analyser (ZVB20, Rohde & Schwarz GmbH & Co KG, Germany) and the
total efficiency was measured in a SATIMO Starlab 16 anechoic chamber under
free-space conditions. To study the effect of humidity, antennas with and without a
conformal coating (Fine-L-Kote SR, Techspray, Kennesaw, GA, USA) were placed
in an environmental chamber (VCL 4006, Vötsch Industrietechnik GmbH,
Germany) and exposed to relative humidities (RH) of 40, 60, and 80% at T = 25°C
for 1 hour.
30
31
4 Room-temperature fabrication method
Heat typically provides the energy needed for the densification in a sintering
process and only a few other methods have been reported. Cold sintering is based
on the plastic deformation of powder particles in a high pressure gradient at ambient
temperature [42]. In addition to metals, it can be applied to ionic or ionic-covalent
solids as long as the pressure during compaction exceeds the flow stress of the
powder particles. Subsequent sintering of the cold sintered compacts at
temperatures significantly lower than those used in conventional sintering results
in high strength and improved ductility. However, the compaction pressure that is
needed to achieve a green density close to the theoretical one is at least 3 GPa. In
order to withstand a pressure this high, very carefully designed tool steel dies are
demanded with an outer diameter approximately three times larger than the part
diameter. For platelet-shaped hydrogen uranyl phosphate tetrahydrate two room-
temperature densification mechanisms have been presented [43]. A pressure-free
solution-phase sintering, where the transport of material occurs through the
solution phase by virtue of the ionic concentration established by the solubility
equilibrium, and a pressure-induced (20 MPa) densification of completely dry
powder by plastic flow in the crystallites by dislocation movement. However, these
two mechanisms do not benefit from combining them: during pressing the presence
of a solution phase greatly retarded densification. [43]
The developed room-temperature fabrication method for Li2MoO4 ceramics
(Papers I–III) is based on the water-solubility of Li2MoO4. The method utilizes a
small amount of aqueous phase formed by moistening the Li2MoO4 powder with
deionized water. Densification occurs during sample pressing as the solution
incorporates the pores between the powder particles and recrystallizes. In an earlier
study conducted with water soluble sodium chloride crystals, it was noted that when
in contact with a solvent under conditions of applied compressive stress, the
solubility increased in the zone of high pressure and that there was a resultant
transfer of material by solution, diffusion through the liquid, and precipitation at
the sites with low pressure conditions [44]. It is hypothesized that in the room-
temperature fabrication method, the contact points of the Li2MoO4 particles provide
the high pressure zone during sample pressing, whereas the pores can act as a
suitable place for the precipitation of the aqueous solution. To remove any residual
water related to the aqueous phase from the samples, they were post-processed
typically at 120 °C for 24 hours. There was no detectable change in the sample
dimensions at this processing step, further proving that the densification does not
32
occur because of the capillary pressure as in conventional liquid phase sintering
[21, 22], but due to the applied pressure. The unaltered dimensions after sample
pressing can be considered a major advantage, since the size of the components can
easily be managed by controlling the size of the mould and the amount of the
powder.
The Li2MoO4 ceramics fabricated by the room-temperature method exhibited
εr of 5.1 and a tan δ value of 3.5 x 10-4 at 9.6 GHz (Paper II). These values are very
similar to those measured for the Li2MoO4 ceramics sintered at 540 °C (εr ~5.5, tan
δ ~2.8 x 10-4 at 13.051 GHz) [4].
The XRD pattern (Fig. 2), where all the peaks correspond to Li2MoO4
according to Powder Diffraction File 12-0763, indicates that Li2MoO4 crystallizes
without forming any stable hydrates, as can be expected according to earlier studies
[30, 31]. The crystal structure of the samples remained the same, even after the
surface of the sample was moistened again with deionized water and left to dry at
room temperature. (Paper I)
33
Fig. 2. XRD diffraction pattern of room-temperature fabricated Li2MoO4. All peaks
correspond to Li2MoO4 (PDF 12-0763) (Paper I, published by permission of John Wiley
and Sons).
4.1 The effect of pressure
The application of pressure during sample fabrication is essential to the room-
temperature fabrication method. It had been found earlier that, for aqueous systems
with completely wetting liquids, the solid particles did not come into contact under
capillary pressure as in liquid-phase sintering but remained at a separation of
0.005–0.004 µm due to repulsive forces between particles or to an effectively rigid,
tightly held liquid film [21, 45, 46]. Therefore, external pressure is needed for this
fabrication method.
The amount of pressure applied to the moistened Li2MoO4 powder affected the
density of the samples. With the original pressure of 130 MPa, the density of the
fabricated Li2MoO4 disks varied from 2.6 to 2.8 g/cm3, which is 87–93% of the
theoretical bulk density. (Paper I) This is quite close to the density of 2.895 g/cm3
(95.5%) that Zhou et al. achieved by sintering at 540 °C [4]. As the effect of the
34
applied pressure on the density was further studied, it was noted that as the pressure
increased from 30 MPa to 180 MPa, the density increased from 2.54 g/cm3 to 2.83
g/cm3 (from 84 to 93% of the theoretical bulk density) reaching its maximum value
at the pressure of 150 MPa. (Paper II) During the sample pressing it was noticed
that the pressure at the mould first dropped rapidly, presumably due to the removal
of the excess aqueous phase, and then the decrease of pressure became slower. It is
assumed that the final densification occurred during this slower pressure drop in
the mould due to the increased solution of the particle contact points. This drives
the aqueous phase to the pores between the particles where it is precipitated.
Increasing the pressure in the mould after the pressure drop had no effect on the
density. (Paper I)
4.2 The effect of the amount of water
The effect of the amount of the solution phase of Li2MoO4 was studied by
moistening the powder with different amounts of deionized water. The density of
the samples stayed at the same level even though the water content of the powder
before pressing the samples changed from 2 to 15 weight-%. Therefore, it is
assumed that as long as the powder is moistened, the amount of added water is not
significant. This is probably due to the continuous dissolution and precipitation
reactions during sample pressing, where the water molecules freed during the
precipitation can take part in a new dissolution reaction. The samples pressed from
dry Li2MoO4 powder with the same pressure of 150 MPa typically acquired a
density of about 80% of the theoretical bulk density, leaving the samples still
difficult to handle since no consolidation occurred. (Paper II)
4.3 The effect of post-processing temperature
The post-processing temperature affects the time needed to remove the residual
water from the samples. The amount of water in the freshly pressed samples was
from 2 to 3 weight-%. The effect of residual water was observed as an increase in
the loss tangent values of the samples post-processed at 120 °C for 4 hours and
measured at around 9.6 GHz. (Paper I) As the post-processing time at 120 °C was
increased from 4 to 24 hours, a decrease in the loss tangent value from 0.0007 to
0.0004 was observed at 9.6 GHz. It was also noted that the relative permittivity and
the loss tangent values were the same for the samples fabricated from the powder
with a particle size of <180 µm and post-processed at 540 °C for 2 hours as they
35
were for the samples post-processed at 120 °C for 24 hours. This indicated that the
post-processing temperature is not decisive and can be suitably selected according
to the characteristics of the materials integrated with the Li2MoO4 ceramics
fabricated by this novel method. (Paper II)
4.4 The effect of powder particle size
Particle size has a well-known effect on conventional sintering [12, 21]. A smaller
particle size enhances sintering, resulting in higher density and permittivity, as well
as in lower loss tangent values. With the novel room-temperature fabrication
method this was not the case. At 9.6 GHz, the samples showed a decrease in
permittivity from 5.1 to 4.2, and an increase in loss tangent values from 0.0004 to
0.0032 as the particle size of the used powder decreased from <180 µm to <45 µm.
Small particle sizes complicated the even moistening of the powder, resulting in
clay-like clusters and non-uniform density leading to sample warpage and cracking.
It can be concluded that when fabricating Li2MoO4 disks with the moistening and
pressing method, a larger particle size is advantageous. (Paper II)
4.5 The effect of insoluble additives
The possibility of modifying the dielectric properties of the room-temperature
densified Li2MoO4 ceramics with insoluble additives was studied at first with 10
volume-% inclusions of rutile TiO2 and BaTiO3. (Paper II) At 1 GHz the relative
permittivity was increased from 6.4 to 8.8 for compositions with TiO2 and to 9.7
with BaTiO3. At the same time the loss tangent value increased from 0.0006 to
0.0014 and to 0.011, respectively. According to Guo et al. [19] the temperature
stabilization of Li2MoO4 with rutile requires higher volumes of TiO2 than 10
volume-%. Therefore, it was thought necessary to study the incorporation of large
amounts of insoluble additives to the room-temperature densified Li2MoO4 ceramic
matrix. (Paper III)
In Paper II the Li2MoO4 powder was prepared by grinding with a mortar and
pestle, and then sieving with a mesh size of 180 µm. According to a laser diffraction
based particle size distribution measurement, this powder had a mean particle size
of 87 µm. The particle size of the used rutile TiO2 powder was 0.9–1.6 µm
according to the manufacturer, and the average size of the used BaTiO3 was about
2 µm. As the amount of insoluble additive increased above 10 volume-%, it became
obvious that the particle size of Li2MoO4 powder had a significant effect also on
36
the densification of composites (Paper III). The larger Li2MoO4 particle size used
resulted in an uneven distribution of TiO2 because, as the amount of TiO2 increased,
the surface area of the Li2MoO4 powder was not great enough to accommodate all
of the TiO2 powder particles. This can be seen in Fig. 3 a), where distinctive dark
grey areas of TiO2 are visible. As the powder was sieved with a mesh size of 45 µm,
a mean particle size of 14 µm was measured and about 43% of particles were
smaller than 10 µm. The use of Li2MoO4 with this smaller particle size led to a
more homogeneous distribution of TiO2 (Fig. 3 b), improvements in the attachment
between the particles, and a decrease in the size of voids. (Paper III)
Fig. 3. Backscattered electron images of Li2MoO4–TiO2 ceramic composites with a) 15
volume-% of TiO2 and fabricated from larger particle sized Li2MoO4 powder (sieved with
a mesh size of 180 µm) b) 20 volume-% of TiO2 and fabricated with smaller particle sized
Li2MoO4 powder (sieved with a mesh size of 45 µm) (Paper III, published by permission
of Elsevier).
Problems related to the use of the Li2MoO4 powder with a small particle size were
addressed in the Li2MoO4–TiO2 composite fabrication by providing more solution
phase, which resulted in a thick uniform paste. The paste was then pressed into the
desired form, although fabrication problems such as flashing of the material to the
pistons of the mould still continued. The fabrication difficulties in wet pressing are
well known in ceramic engineering and provide problems in automated pressing
processes [13]. To achieve dense samples with a uniform microstructure, it is clear
that a compromise between the processibility and the particle size of Li2MoO4 is
37
required, especially in the case of high loading levels of an insoluble additive like
TiO2. (Paper III)
An increased amount of aqueous Li2MoO4 solution is also necessary, since
TiO2 is not water soluble. Insoluble particles need to be covered totally with
aqueous Li2MoO4 solution to achieve densification. While in pure Li2MoO4
ceramics the dissolution and recrystallization can occur at all the surfaces and
provide water molecules for continuous dissolution during the sample pressing, this
is not the case with insoluble additives. When the amount of solution phase was
increased with the amount of TiO2 phase, the amount of residual water in the
samples also increased. As the residual water evaporates from the samples during
post-processing, it leaves very small pores in the samples leading to a decrease in
density. The density decreased from 93% to 86% of the theoretical value as the
amount of TiO2 increased from 0 to 30 volume-%. The finely distributed, very small
pores did not seem to affect the processing of the ceramic composite samples, as
they were still hard and durable even with 30 volume-% of TiO2. A decrease in
density has also been observed for sintered Li2MoO4–Ni0.5Zn0.5Fe2O4 composites
(the relative density decreased from 96% to 89% compared with the calculated
theoretical density) since the increased content of the Ni0.5Zn0.5Fe2O4 particles
disturbed the interaction between each of the developing Li2MoO4 grains [34].
The X-ray diffraction patterns of the Li2MoO4–TiO2 ceramic composites in Fig.
4 (a) show only the diffraction lines of Li2MoO4 [Powder Diffraction File 12-0763]
and rutile TiO2 [Powder Diffraction File 34-0180] phases. Guo et al. reported [19]
that when Li2MoO4–TiO2 composites were co-fired with silver (Ag), small amounts
of LiTi2O4 and Ag2MoO4 phases were formed. It is likely that this partial reaction
was caused by the relatively high sintering temperatures of the composites (700–
730°C), which were close to or above the melting temperature of Li2MoO4. Fig. 4
(b) shows the XRD patterns of a similar composite ceramic with Ag fabricated by
the room-temperature fabrication method. Only Li2MoO4, rutile TiO2 and Ag
phases are observed, which confirms that with this novel fabrication method no
reaction between Ag and Li2MoO4–TiO2 composite materials occurs. (Paper III)
38
Fig. 4. X-ray diffraction patterns of (a) fabricated Li2MoO4–TiO2 ceramic composites, and
(b) Li2MoO4–TiO2 ceramic composites with 30 volume-% of TiO2 and 20 weight-% of
silver (Paper III, published by permission of Elsevier).
The dielectric properties of all fabricated Li2MoO4 composites are presented in
Table 1. (Papers II–III) The relative permittivity of the Li2MoO4–TiO2 composites
increased from 6.9 to 10.1 at ~9 GHz as the amount of TiO2 was increased from 10
volume-% to 30 volume-%. This is due to the higher permittivity of rutile TiO2. As
the amount of TiO2 phase exceeded 20 volume-%, the correspondence between the
Lichtenecker equation (2) and the measurements of the composites decreased due
39
to the increased porosity. (Paper III) The measured relative permittivities of
sintered Li2MoO4–TiO2 composites were generally in accordance with the
Lichtenecker equation for a two-phase composite [19]. The dielectric loss values
of rutile TiO2 and Li2MoO4 were of the same magnitude (10-4). The measured loss
values of the composites were therefore higher than expected, rising at ~9 GHz
from 1.1×10-3 to 3.8×10-3 as a function of the amount of TiO2. Such behaviour has
also been reported by Guo et al. [19] with sintered composites, and it was assumed
to be due to the difference in the sintering temperatures of the two phases. For the
room-temperature fabricated samples, the increased porosity is a plausible cause
also for the increase in dielectric losses.
Table 1. Dielectric properties of all the fabricated Li2MoO4 ceramic composites.
Composition εr tan δ TCεr Paper
1 GHz 9.1–9.6 GHz 1 GHz 9.1–9.6 GHz
Li2MoO4 6.4 5.1 0.0006 0.0004 320 II, III
+10 vol% BaTiO3 9.7 8.2 0.011 0.026 n/a II
+10 vol% TiO2 8.8 6.7–6.9 0.0014 0.0011–0.0014 180 II, III
+15 vol% TiO2 n/a 7.2 n/a 0.0020 125 III
+20 vol% TiO2 n/a 8.7 n/a 0.0023 20 III
+25 vol% TiO2 n/a 9.5 n/a 0.0027 -85 III
+30 vol% TiO2 n/a 10.1 n/a 0.0038 -170 III
40
41
5 The applicability of room-temperature fabricated Li2MoO4 ceramics for antennas
A particular advantage of the room-temperature fabrication method is the ease of
managing the size of the final ceramic product by controlling the mould dimensions.
Therefore, the method is favourable for the design of size sensitive ceramic
components such as antennas. Antennas are required to operate reliably under
varying temperature and humidity conditions. For this reason, the temperature
stabilization of Li2MoO4 ceramics such as those fabricated by the room-
temperature method is necessary. It is also well known that the adsorption of water
molecules on the surface of oxides changes the dielectric properties of ceramics.
[47] Therefore the stability under changing humidity should also be studied.
A ceramic patch antenna was realized utilizing a room-temperature densified
Li2MoO4 ceramic component. In Fig. 5 (a) the layout and cross section of the
designed antenna, and (b) a photographic image of a typical fabricated antenna are
presented.
Fig. 5. a) Layout and cross-section of the designed antenna, and b) image of a typical
fabricated circular Li2MoO4 patch antenna (Paper IV).
42
Fig. 6 shows the simulated and measured (a) return loss values (S11), (b) total
efficiencies, and (c) radiation efficiencies of a typical fabricated Li2MoO4 circular
patch antenna. According to the simulations of the antenna structure, the peak
resonance frequency (f0) was 3.98 GHz. The fabricated antennas showed values of
f0 from 3.97 to 4.01 GHz, related to fabrication tolerances. The variation in f0 was
also the reason for the difference between simulated and measured total efficiency.
Radiation efficiency showed a good match between the simulated and measured
results. Measurement results demonstrated that Li2MoO4 ceramics fabricated by
the room-temperature method are feasible in patch antenna design. (Paper IV)
43
Fig. 6. Simulated and measured (a) return loss values (S11) (b) total efficiencies, and (c)
radiation efficiencies of a typical fabricated circular Li2MoO4 patch antenna (Paper IV).
44
5.1 The effect of humidity
When water vapour chemisorbs on a bare oxide surface by a dissociative
mechanism it forms two surface hydroxyls for each water molecule. Subsequent
layers of water molecules then adsorb physically on the hydroxyl layer. The first
layer of physisorbed water molecules is localized rigidly by double hydrogen
bonding. Therefore, its effect on the relative permittivity is insignificant. As the
relative humidity increases and the physisorption changes from monolayer to
multilayer, water molecules are only singly bonded and therefore form dipoles.
Since dipoles can reorient freely under an externally applied electric field, the
physisorbed multilayer of water molecules results in a change in dielectric
properties. [47] The water-solubility of Li2MoO4 raises questions about its stability
under exposure to humidity and therefore it was studied in the antenna application
without any insoluble additives. Typically, moisture sensitive components may be
protected by using an electronic conformal coating. According to a previous study
[48], a silicone elastomer has only a low tendency to absorb moisture.
Fig. 7 illustrates the effect of humidity on (a) return loss, (b) total efficiency,
and (c) radiation efficiency of an unprotected circular Li2MoO4 patch antenna. It
may be noted that, while low values of relative humidity (RH) had little effect on
the characteristics, a RH of 80% had a notable effect on the f0 and on the total and
radiation efficiencies of the antenna. The decrease in the f0 was about 130 MHz,
3.25% of the original resonance frequency value, and it is related to the increase in
the permittivity of the ceramic due to the physisorbed water with εr ~76 [49].
Absorbed water also increased the dielectric loss value of Li2MoO4 (Paper I)
causing a further decrease in the total and radiation efficiencies of the antenna. It is
also possible that the porosity of the ceramic (density 93% of the theoretical value)
allowed water to condense in the capillary-like pores between the grain surfaces
[50]. Furthermore, due to the variations in the fabrication of the antennas, there
might have been some condensed water in the gap between the PCB and the
ceramic. After the RH was decreased from 80% back to 29% (ambient RH), and
the sample was left for 18 hours, the effects of the high RH level on the f0 and
efficiency were observed to gradually reverse. This indicated a slow rate of
desorption of water from the antenna. (Paper IV)
45
Fig. 7. The effect of relative humidity (RH) on (a) return loss, (b) total efficiency, and (c)
radiation efficiency of a typical fabricated circular Li2MoO4 patch antenna (Paper IV).
46
The applied silicone coating had only a very small effect on the f0 of the antennas,
decreasing it at most by 20 MHz (about 0.5% of the original value) and it had no
effect on the efficiencies. Fig. 8 shows the effects of different RH levels on return
loss, total efficiency, and radiation efficiency of a Li2MoO4 patch antenna with a
moisture protective silicone conformal coating. The silicone coating clearly
reduced the effects related to the increased humidity. As the RH increased from
26% to 80%, the resonance frequency decreased only about 50 MHz, which
corresponds to a 1.26% shift from the original value. Also, the decrease in the total
and radiation efficiencies was smaller than in the case of the unprotected antenna.
The decreases were completely reversible, indicating that if the antenna is protected,
the effect the humidity has on the antenna is related to the water adsorbed on the
surface of the conformal coating. This physisorbed water was easily desorbed as
the RH decreased. Consequently, the room-temperature densified Li2MoO4
ceramics are feasible to use under high relative humidity conditions if a protective
conformal coating is applied. (Paper IV)
47
Fig. 8. The effect of relative humidity (RH) on (a) return loss, (b) total efficiency, and (c)
radiation efficiency of a typical fabricated circular Li2MoO4 patch antenna with silicone
conformal coating (Paper IV).
48
5.2 The effect of temperature
Temperature stabilization of Li2MoO4 with rutile TiO2 has been implemented by
Guo et al. by the fabrication of sintered microwave dielectric compositions
0.55Li2MoO4–0.45TiO2 (Ts = 700 °C) and 0.50Li2MoO4–0.50TiO2 (Ts = 720 °C)
[19]. For the Li2MoO4–TiO2 composites fabricated by the room-temperature
method, an increasing amount of TiO2 addition decreased the value of TCεr, as
shown in Fig. 9, and it was closest to zero (~20 ppm/°C) with a loading level of 20
volume-%. Fig. 9 confirms the feasibility of the room-temperature fabrication
method also for the temperature stabilization of Li2MoO4 ceramics with a large
amount of insoluble additives. Furthermore, the dielectric properties of the room-
temperature fabricated Li2MoO4–TiO2 composites are similar to those of
commercially available LTCC substrates from Ferro, Heraeus and CeraTec (εr 5.9–
8.5 and tan δ values up to 0.0026). These substrates have previously been reported
as feasible for ultra-wideband (UWB) antennas operating in the frequency range of
3.1-10.6 GHz. [51–53] Therefore the Li2MoO4–TiO2 composites fabricated in this
work are also good candidates, for example, for UWB antennas. (Paper III)
49
Fig. 9. Calculated and measured relative permittivity, loss tangent values and
temperature coefficient of permittivity of the Li2MoO4–TiO2 composite ceramics
fabricated at room temperature at ~9 GHz. Composites with 10–15 volume-% and 20–30
volume-% of TiO2 were fabricated from Li2MoO4 powder sieved with mesh sizes of 180
µm and 45 µm, respectively (Paper III, published by permission of Elsevier).
50
51
6 Summary and conclusions
Microwave dielectric ceramics are commonly used in portable devices with
wireless communication technologies because their properties enable the
miniaturization of several components. However, conventionally they require a
high sintering temperature to achieve their optimal properties.
This thesis presents a method for the fabrication of relatively dense ceramics
at room-temperature with competent microwave dielectric properties. The method
utilizes a small amount of water with Li2MoO4 powder and the densification occurs
during sample pressing. To remove any residual water molecules, the samples are
typically post-processed at 120 °C. The following advantages were also noted:
– The dielectric properties of this Li2MoO4 ceramic can be optimized by the
composite technology enabling applications, for example, in electronics
packaging and wireless communication.
– The post-processing temperature of the fabricated ceramics can be chosen to
be suitable to the associated integrated materials, for example the electrode
material, as long as the post-processing time is sufficient to ensure the removal
of the residual water.
– The low fabrication temperature can prevent the high-temperature induced
formation of unwanted extra phases with the electrode material or additives,
which can be used to modify the properties of the Li2MoO4 ceramics.
– The size of the ceramic component is easily controlled by managing the size
of the mould and the amount of the ceramic powder. This is an important
advantage in the fabrication of size sensitive ceramic component applications,
such as antennas.
– The method offers new possibilities for the seamless integration of ceramic
components with temperature-sensitive substrate materials such as polymers
or paper.
The applicability for antenna design of the ceramics fabricated by the room-
temperature method was also studied. The results showed that temperature
stabilization of these ceramics could be achieved by the fabrication of Li2MoO4–
TiO2 composites. The dielectric properties of fabricated composites were similar to
those of commercial substrates that have been reported to be suitable for antennas
operating at microwave frequencies. A high humidity level slightly affected the
peak resonance frequency of a Li2MoO4 patch antenna and decreased its total and
52
radiation efficiencies. A silicone conformal coating reduced these changes and
expedited their reversibility when the humidity level was again lowered. However,
in the future the feasibility of this coating should also be studied for an antenna
realized with a Li2MoO4–TiO2 ceramic patch.
Some other future challenges also exist. The thermal conductivity and the
mechanical properties, which were outside the scope of this work, should be
examined. Also, the manufacturing of ceramic multilayer components would
require a totally new approach, not only because the typical additives (binders,
dispersants and plasticizers) used in the ceramic tape casting are not suitable for
the room-temperature fabrication, but also because the driving force of the room-
temperature densification is pressure, not heat, as in conventional sintering.
However, other fabrication methods such as 2D and 3D printing already provide
interesting new options for components and modules, especially if integration with
polymers and other temperature-sensitive materials is needed. It is also plausible
that the method is suitable for other similar ceramic materials, providing even more
possibilities to utilize the method in advanced packaging and in the fabrication of
composites.
53
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Original papers
I Kähäri H, Teirikangas M, Juuti J & Jantunen H (2014) Dielectric properties of lithium molybdate ceramic fabricated at room temperature. Journal of the American Ceramic Society 97(11): 3378–3379.
II Kähäri H, Teirikangas M, Juuti J & Jantunen H (2015) Improvements and modifications to room-temperature fabrication method for dielectric Li2MoO4 ceramics Journal of the American Ceramic Society 98(3): 687–689.
III Kähäri H, Teirikangas M, Juuti J & Jantunen H (2016) Room-temperature fabrication of microwave dielectric Li2MoO4–TiO2 composite ceramics. Ceramics International 64(1): 71–75.
IV Kähäri H, Ramachandran P, Juuti J & Jantunen H (2016) Room-temperature densified Li2MoO4 ceramic patch antenna and the effect of humidity. Manuscript.
Reprinted with permissions from John Wiley and Sons (I, II) and Elsevier (III).
Original publications are not included in the electronic version of the dissertation.
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