UNIVERSITATIS OULUENSIS ACTA C TECHNICA OULU 2016 C 584 Hanna Kähäri A ROOM-TEMPERATURE FABRICATION METHOD FOR MICROWAVE DIELECTRIC Li 2 MoO 4 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 Kähäri
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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
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.
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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17
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
[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)
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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).
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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
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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.
58
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