Conclusions As a summary, high quality epitaxial or textured ferroelectric and dielectric thin films, including BST both single layer and nanostructured multilayer, PZT, and CCT, have be
Trang 2BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 25
Freq ue nc y (G Hz) -7
-6 5 -6 -5 5 -5 -4 5 -4 -3 5 -3 -2 5 -2 -1 5 -1
DB (|S(2 ,1 )|) 0V
2 5V DB( |S (2,1)|)
3 0V DB( |S (2,1)|)
3 5V
Freq uency (G Hz) -4 0
-3 5 -3 0 -2 5 -2 0 -1 5 -1 0 -5 0
3 GH z -11.53 dB
DB (|S(1, 1)| ) 0V
DB (|S(1, 1)| ) 05V
DB (|S(1, 1)| ) 10V
DB(|S(1,1)|) 15V DB(|S(1,1)|) 20V DB(|S(1,1)|) 25V
D B(|S(1,1)|) 30V
D B(|S(1,1)|) 35V
Fig 30 (a) Insertion loss, S21 and (b) return loss, S11 of 3 GHz phase shifter
Frequency (GH z) 0
20 60 100 140 180 220 260 300 340 380
Fig 31 Phase shift of the 3 GHz phase shifter at different frequencies and bias voltages
(a)
(b)
Trang 35 Conclusions
As a summary, high quality epitaxial or textured ferroelectric and dielectric thin films, including BST (both single layer and nanostructured multilayer), PZT, and CCT, have been successfully deposited by the proprietary CCVD process onto various substrates, including sapphire and single crystal STO, MgO, and LAO etc Excellent electrical properties have been achieved on these ferroelectric and dielectric thin films High performance microwave devices that can be used up to Ka band, such as tunable MEMS filters and CDMA filters, have been designed and fabricated on BST based ferroelectric thin films The performance of these microwave devices are summarized as following:
MEMS Ka-band tunable bandpass filters (both center frequency and bandwidth are tunable): the best insertion loss of 3 dB when biased, and the bandwidths of 3 and 7.8% for 3-pole narrowband and wideband, respectively;
CDMA Tx tunable filters: insertion loss <2 dB, VSWR <1.5:1, center frequency shifting from 1.85 to 1.91 GHz, Rx zero (@1.93 GHz) rejection >40 dB, DC bias <10 V;
X- to Ku-band tunable bandpass filters: insertion loss of ~5 dB @11.5 GHz (0V) to 3 dB
@14 GHz (30 V), VSWR <2:1, DC bias <30 V, 6 × 1.5 × 0.5 mm in footprint;
X-band back-to-back 4-pole bandpass filters: Insertion loss from 5.4 dB at 9.1 GHz to 1.84 dB at 10.25 GHz with an analog tuning of 12.6%; return loss <10 dB over the whole X-band frequency range;
Ka-band ring filters: insertion loss of 2.3 and 2.0 dB for 0 and 30 V, respectively; 3-dB bandwidth of 20% for both bias states; tuning from 31.6 to 33.7 GHz, a 6.3% tunability;
3 GHz phase shifter: The insertion loss at is 4.3 dB at 15 V and 3 GHz The figure of merit is 89.4°/dB at 0V A phase shift of 361° is measured at 30V
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Trang 82
(Molten Salt Synthesis) Method
is necessary to develop environment – friendly lead – free piezoelectric ceramics to replace the PZT – based ceramics, which has become one of the main trends in present development
of piezoelectric materials Sodium bismuth titanate, Na0.5Bi0.5TiO3 (abbreviate as NBT), discovered in 1960 (Smolenskii et al., 1960), is considered to be a promising candidate of lead – free piezoelectric ceramics (Pookmaneea et al., 2001; Isupov, 2005; Panda, 2009; Zhou
et al., 2010)
NBT is a relaxor ferroelectric material with the general formula A’xA”1-xBO3 The ferroelectricity in NBT ceramic is attributed to (Bi1/2Na1/2)2+ ions, especially Bi3+ ions at the ,,A” site of the perovskite structure (ABO3) and due to rhombohedral symmetry at room temperature It has high Curie temperature (Tc = 320°C), and shows diffuse phase transition (Suchanicz et al., 2000; Suchanicz et al., 2001; Raghavender et al., 2006) However, the piezoelectric properties of NBT ceramics are not good enough for most practical uses In order to enhance the properties and meet the requirements for practical uses, it is necessary
to develop new NBT – based ceramics (Raghavender et al., 2006; Zhou et al., 2010) Researches have investigated many dopants into NBT ceramics (Panda, 2009) Also, it is desirable to fabricate ceramics with a textured microstructure in order to improve the properties (Hao et al., 2007)
The ferroelectric ceramic powders are synthesized trough conventional solid – state method which needs high calcination temperature and repeated grindings (Lu et al., 2010) In order to eliminate these defects, the wet chemical synthesis techniques have been developed, for instance hydrothermal method (Cho et al., 2006; Wang et al., 2009), sol – gel method (Xu et al., 2006; Mercadelli et al., 2008), and molten salt method (Zeng et al., 2007; Li et al., 2009) But the hydrothermal and sol – gel synthesis are usually long and complex processes, use hazardous solvents such as 2-methoxyethanol, and result in agglomerated particles (Bortolani & Dorey, 2010) Moreover, in the sol –gel method the cost of starting materials is high (Li et al., 2009)
Trang 9Molten salt synthesis (MSS) is a process that yields large amounts of ceramic powders in a
relatively short period of time Moreover, it is a suitable method for preparation of complex
oxide compounds with anisotropic particle morphologies In this technique starting materials
are mixed together with a salt (usually alkaline chloride and sulphate) and then heat treated at
a temperature higher than the melting point of the salt The melting temperature of the salt
system can be reduced by using a eutectic mixture of salts, e.g the use of NaCl – KCl instead of
pure NaCl reduces the melting point from 801 to 657°C A reaction between the precursors
takes place in the molten salt (the flux) and the solid product obtained is separated by washing
of the final mixture with hot deionised water The typical starting materials are oxides, but
carbonates, oxalates and nitrates can also be used There are several requirements for the
selection of salt to be used for MSS First, the melting point of the salt should be relatively low
and appropriate for synthesizing of the required phase Second, the salt should possess
sufficient aqueous solubility in order to eliminate it easily after synthesis by washing Finally,
the salt should not react with the starting materials or the product (Bortolani & Dorey, 2010;
Hao et al., 2007) MSS has been used to form various ceramic powders such as niobates
relaxors (Yoon et al., 1998), Bi4Ti3O12 (Kan et al., 2003), ZnTiO3 (Xing et al., 2006), BaTiO3
(Zhabrev et al., 2008) and Pb(Zr, Ti)O3 (Cai et al., 2008; Bortolani & Dorey, 2010)
It was found that ternary compound Na0.5Bi0.5TiO3 was formed in the solid – state process
through the intermediate binary compound, i.e bismuth titanate – Bi4Ti3O12 (Zaremba,
2008) Bi4Ti3O12 (abbreviate as BIT) belongs to the Aurivillius family with a general formula
(Bi2O2)[Am-1(B)mO3m+1], which consists of (Bi2O2)2+ sheets alternating with (Bi2Ti3O10)2
-perovskite – like – layers (Aurivillius, 1949, as cited in Stojanović et al., 2008) In general
formula m represents the number of octahedra stacked along the direction perpendicular to
the sheets, and A and B are the 12- and 6- fold coordination sites of perovskite slab,
respectively This kind of structure promotes plate – like morphology (Dorrian et al., 1971,
as cited in Stojanović et al., 2008)
In this paper, Na0.5Bi0.5TiO3 powders were prepared by molten salt synthesis in the presence
pure NaCl or NaCl - KCl as fluxes The first stage of the study related to direct synthesis of
NBT via MSS from Na2CO3, Bi2O3 and TiO2 For comparison, the synthesis of NBT by the
conventional method (CMO – conventional mixed oxides) was investigated The second
stage included obtaining intermediate binary compound BIT via MSS from oxide
precursors, i.e Bi2O3 and TiO2, and then synthesis of NBT via MSS using BIT, Na2CO3 and
TiO2 as starting materials
The details pertaining to studies of synthesis of NBT and an Aurivillius – structured BIT
precursor are reported in the following sections
2 Synthesis of ferroelectric Na0.5Bi0.5TiO3
Chemically pure powders of Bi2O3, TiO2 (rutile) and Na2CO3 were used as starting
materials The Na0.5Bi0.5TiO3 (NBT) was prepared by the following two routes:
Bi4Ti3O12 + 5TiO2 + 2Na2CO3 → 8Na0.5Bi0.5TiO3 + 2CO2 (2)
In route (1), the starting materials were weighed in the proportion to yield NBT and mixed
in isopropyl alcohol employing an agate mortar and pestle for 1 h Using MSS method, the
dry mixture of the precursors in the stoichiometric composition was mixed with an aqual
Trang 10Synthesis of Ferroelectric Na 0.5 Bi 0.5 TiO 3 by MSS (Molten Salt Synthesis) Method 33 weight of salt Salts used in this experiment were NaCl and eutectic mixture of 0.5NaCl – 0.5KCl, i.e 43.94% NaCl – 56.06% KCl (by weight) The mixture of the precursors and flux was dried at 120°C for 2 h for complete removal of isopropyl alcohol, placed in a Pt crucible and heated in a sealed alumina crucible (to prevent salt evaporation) at temperatures between 800°C and 1100°C for a different time period After thermal treatment the chlorides were removed from the products by washing with hot deionized water several times until the filtrates gave no reaction with silver nitrate solution The powders were finally dried at
100°C for 2 h NBT powders were also prepared by a conventional mixed oxide method (CMO) for comparison All the syntheses were carried out in a conventional electric furnace Platelike Bi4Ti3O12 (BIT) particles were obtained by MSS method in 0.5NaCl – 0.5KCl flux (abbreviate as NaCl – KCl) in the same way as described above Temperature of thermal treatment ranging from 700°C to 1100°C for a different time period
In route (2), BIT crystals produced earlier were subjected to second molten salt synthesis
Na2CO3 and TiO2 were added to give the total composition of NBT Again, pure NaCl or NaCl – KCl mixture was added (weight ratio of precursors to flux = 1:1)
Finally, the phase composition of the synthesized samples was analyzed by the powder ray diffraction (XRD; model 3003 TT, Seifert) using Ni – filtered Cu Kα radiation The microstructure was observed by a scanning electron microscope (SEM; model BS 340, Tesla) The samples were coated by a gold layer by using a metal – coating plant under a vacuum X–ray energy dispersive spectra (EDS) were measured using a Hitachi S-3400 N scanning electron microscope with an EDS system Thermo Noran
X-2.1 Synthesis of Na 0.5 Bi 0.5 TiO 3 from Bi 2 O 3 , TiO 2 and Na 2 CO 3
Fig 1 represents the XRD patterns of the selected powders synthesized through route (1), i.e directly from Bi2O3, TiO2 and Na2CO3 via MSS (NaCl flux) and CMO Similar trends were also observed for NBT produced using NaCl- KCl flux The particle morphology of the starting materials and synthesized powders is compared in Figs 2 – 4
NBT perovskite phase was observed in all the prepared samples A comparison of interplanar spacings determined from XRD patterns of the samples prepared by a conventional solid state reaction and via MSS shows that agreement is quite satisfactory Analysis of XRD patterns of NBT samples obtained via MSS has not shown displacement of maxima of diffraction peaks as the NaCl-KCl flux was used
Isometric particles are found to exist in the samples of NBT Typical micrograph of the NBT powder prepared by CMO is shown in Fig 3a There is high degree of agglomeration in this powder The NBT particles prepared directly by CMO and MSS (NaCl flux) are very small (about 1 μm) The size of the particles increased with increasing temperature, especially, as NaCl-KCl flux was used Probably, this is mainly due to the different melting points for each salt used NaCl and 0.5 NaCl – 0.5 KCl have melting points of about 800°C and 650°C, respectively
According to (Cai et al., 2007, as cited in Bortolani & Dorey, 2010) the solubility of the starting materials in the molten salt plays an important role in the synthesis as it has an influence on the final product morphology For a simple two reactant system, two different cases can be distinguished: either both reactants are equally soluble in the molten salt or one oxide is more soluble than the other (Li et al., 2007, as cited in Bortolani & Dorey, 2010) In the first case (dissolution – precipitation mechanism) both reactants fully dissolve, react in the molten salt and the final product precipitates from the molten salt after formation The shape of the product has no connection with the shape of the starting materials In the second case, the more soluble precursor dissolves in the salt and diffuses to the less soluble
Trang 11one Here, at the surface, it reacts to final product This template formation mechanism would result in a product morphology that is similar to that of the less soluble reactant which has acted as a template
Fig 1 XRD patterns of Na0.5Bi0.5TiO3 powders fabricated through route (1): (a) via MSS (NaCl flux) at 950°C for 1.5 h; (b) via MSS (NaCl flux) at 1000°C for 4 h; (c) via CMO at
1000°C for 4 h
Trang 12Synthesis of Ferroelectric Na 0.5 Bi 0.5 TiO 3 by MSS (Molten Salt Synthesis) Method 35
(a)
(b)
(c) Fig 2 SEM micrographs of the starting materials: (a) Na2CO3; (b) Bi2O3; (c) TiO2 - rutile
Trang 13(a)
(b)
(c) Fig 3 SEM micrographs of Na0.5Bi0.5TiO3 powders obtained through route (1): (a) via CMO
at 950°C for 1.5 h.; (b) via MSS (NaCl flux) at 950°C for 1.5 h; (c) via MSS (NaCl flux) at 1000°C for 4 h
Trang 14Synthesis of Ferroelectric Na 0.5 Bi 0.5 TiO 3 by MSS (Molten Salt Synthesis) Method 37
(a)
(b)
(c)
(d) Fig 4 SEM micrographs of Na0.5Bi0.5TiO3 powders obtained through route (1) via MSS (NaCl - KCl flux) at different temperatures for 4 h: (a) 800°C; (b) 900°C; (c) 1000°C; (d)
1100°C
Trang 15In the case of NBT the mechanism is further complicated due to the presence of (at least) 3 reactants According to (Cai et al, 2007, as cited in Bortolani & Dorey, 2010) TiO2 is not soluble in molten alkali chlorides The final NBT morphology should be similar to the morphology of TiO2
Fig 5 XRD patterns of Bi4Ti3O12 powders obtained via MSS at: (a) 700°C for 15 min; (b)
900°C for 30 min; (c) 900°C for 240 min; (d) 1000°C for 15 min (indexed peaks are those of
Bi4Ti3O12)
Trang 16Synthesis of Ferroelectric Na 0.5 Bi 0.5 TiO 3 by MSS (Molten Salt Synthesis) Method 39
(a)
(b)
(c)
(d) Fig 6 SEM micrographs of Bi4Ti3O12 powders obtained via MSS at 700°C for: (a) 15 min; (b)
30 min; (c) 60 min; (d) 120 min
Trang 17(a)
(b)
(c)
(d) Fig 7 SEM micrographs of Bi4Ti3O12 powders obtained via MSS at 1000°C for: (a) 15 min; (b)
30 min; (c) 60 min; (d) 120 min