Microwave Properties

The objects of our research is magnesium aluminates (spinel), in which the electric charge of Al3+ cation is partially substituted with constitutive trivalent cation of Mg2+ and Ti 4+ in the system with the following composition: Mg1+xTixAl2-2xO4 with (x) varying from 0,1 to 0,6. X-ray diffraction data shows that within the above values of (x) the composition is a single phase spinel. The microwave basic parameters were measured as εr, tan δ and τf, respectively with the following values: εr = 6-12, tanδ = (1.5-3.5) x 10-4 at 10 GHz and Qf = 70 000 – 90 000, and τf from -50 to -70 ppm/deg.

We report results on the structural and microwave properties and magnetic phase transitions in polycrystalline Sr3Co2Fe24O41 hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave absorbing material. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves revealed a change in the magnetization behavior at 293 K. The reflection losses in the 1–20 GHz range of the Sr3Co2Fe24O41 powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in both the absence and presence of a magnetic field. In the latter case, a dramatic rise in the attenuation was observed. The microwave reflection losses reached the maximum value of 32.63 dB at 17.29 GHz in the Ku-band. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by the appearance of new reflection losses maxima. At a fixed thickness tm of the composite, the attenuation peak frequency can be adj...

The microwave transmission properties of printed graphene are studied by measuring different length graphene coplanar wave guides up to 10 GHz and by modelling this behaviour with simple transmission line models. The graphene coplanar wave guide is printed on a PET ST 506 flexible substrates by screen printing method. Vorbeck inks S301 were used in the printing process. 10 and 30 mm long graphene transmission line samples were attached on FR4 test boards and measured. These two graphene lines were modelled with lumped equivalent circuits with scalable parameters. Two equivalent circuit approaches were compared and the high frequency fits suggest that a simple model for graphene conductivity is not sufficient. High frequency behaviour is not possible to model with a bulk conductor but a capacitive path has to be added lowering the high frequency conductivity. This is probably a result of the flake structure of printed graphene.

This paper reports the synthesis, structural characteristics and magnetism of SrFe 12 O 19 /MCe 0.04 Fe 1.96 O 4 (M = Cu, Ni, Mn, Co and Zn) hard/soft nanocomposites. The hard/soft compositions were manufactured via a one-pot reactions citrate sol-gel approach. The hard/soft phases formation was confirmed using XRD, SEM, TEM and HRTEM techniques. M vs. H (Magnetization measurements) were done at unbent temperature and 10 K. Smoothed M against H loops and single peaks in dM/dH vs. H curves were noticed

Ba 5 Li 2 W 3 O 15 (BLWO) electroceramics with different amounts of lithium carbonate (10-15%) were synthesized using the solid-state route. Powder x-ray diffraction (PXRD) and Raman spectroscopy showed the presence of the BaWO 4 (BWO) phase in the samples. The excess lithium in the BLWO matrices contributed to an increase in the BWO phase and improved the thermal stability. Images obtained by scanning electron microscopy (SEM) showed grains without a well-defined morphology and with a low porosity. The dielectric properties in the microwave (MW) range for the dielectric resonator based on BLWO were analysed by the Hakki-Coleman and Silva-Fernandes-Sombra methods. The near-zero temperature coefficient of resonant frequency (6.59 ppm°C −1) was obtained for the sample synthesized with 15% Li 2 CO 3. Numerical studies based on the coefficient of reflection (S 11) and impedance values of the BLWO-based dielectric resonator antennas (DRA) were also conducted. They agreed well with the experimental results, which indicates that this material can be applied as an antenna device in the MW range.

The thesis describes electrically tunable microwave devices utilising low sintering temperature, screen printable Barium Strontium Titanate (BST) thick films. The work has been divided into two parts. In the first section, the fabrication and microwave characterisation of BST material based structures compatible with Low Temperature Cofired Ceramic technology (BST-LTCC) are presented. Three different fabrication techniques, namely: direct writing, screen printing and via filling techniques, were used for the realisation of the structures. A detailed description of these fabrication techniques is presented. The dielectric properties such as relative permittivity, static electric field dependent tunability and loss tangent of BST-LTCC structures at microwave frequencies were characterised using coplanar waveguide transmission line and capacitive element techniques. The measured dielectric properties of BST-LTCC structures realised with the different fabrication methods are presented, ...

The thesis describes electrically tunable microwave devices utilising low sintering temperature, screen printable Barium Strontium Titanate (BST) thick films. The work has been divided into two parts. In the first section, the fabrication and microwave characterisation of BST material based structures compatible with Low Temperature Cofired Ceramic technology (BST-LTCC) are presented. Three different fabrication techniques, namely: direct writing, screen printing and via filling techniques, were used for the realisation of the structures. A detailed description of these fabrication techniques is presented. The dielectric properties such as relative permittivity, static electric field dependent tunability and loss tangent of BST-LTCC structures at microwave frequencies were characterised using coplanar waveguide transmission line and capacitive element techniques. The measured dielectric properties of BST-LTCC structures realised with the different fabrication methods are presented, compared and discussed. The second section describes tunable microwave devices based on BST-LTCC structures. A frequency tunable folded slot antenna (FSA) with a screen printed, integrated BST varactor is presented. The resonant frequency of the FSA was tuned by 3.2% with the application of 200 V external bias voltage. The impact of the BST varactor on the total efficiency of the antenna was studied through comparison with a reference antenna not incorporating the BST varactor. A compact, frequency tunable ceramic planar inverted-F antenna (PIFA) utilising an integrated BST varactor for mobile terminal application is presented. The antenna's resonant frequency was tuned by 3% with an application of 200 V bias voltage. Frequency tunable antennas with a completely integrated electrically tunable BST varactor with silver metallisation are introduced in this work for the first time. The integration techniques which are described in this thesis have not been previously reported in scientific literature. The last part of the thesis presents a microwave delay line phase shifter operating at 3 GHz based on BST-LTCC structures. The figure of merit (FOM) of the phase shifter was measured to be 14.6 °/dB at 3 GHz and and the device employs a novel structure for its realisation that enabled the required bias voltage to be decreased, while still maintaining compliance with standard screen printing technology. The performance of the phase shifter is compared and discussed with other phase shifters realised with the BST thick film process. The applications of BST-LTCC structures were demonstrated through frequency tuning of antennas, varactors, and phase shifters. The low sintering temperature BST paste not only enables the use of highly conductive silver metallisation, but also makes the devices more compact and monolithic.

The objects of our research is magnesium aluminates (spinel), in which the electric charge of Al3+ cation is partially substituted with constitutive trivalent cation of Mg2+ and Ti 4+ in the system with the following composition: Mg1+xTixAl2-2xO4 with (x) varying from 0,1 to 0,6. X-ray diffraction data shows that within the above values of (x) the composition is a single phase spinel. The microwave basic parameters were measured as εr, tan δ and τf, respectively with the following values: εr = 6-12, tanδ = (1.5-3.5) x 10-4 at 10 GHz and Qf = 70 000 – 90 000, and τf from -50 to -70 ppm/deg.

We report results on the structural and microwave properties and magnetic phase transitions in polycrystalline Sr3Co2Fe24O41 hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave absorbing material. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves revealed a change in the magnetization behavior at 293 K. The reflection losses in the 1–20 GHz range of the Sr3Co2Fe24O41 powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in both the absence and presence of a magnetic field. In the latter case, a dramatic rise in the attenuation was observed. The microwave reflection losses reached the maximum value of 32.63 dB at 17.29 GHz in the Ku-band. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by the appearance of new reflection losses maxima. At a fixed thickness tm of the composite, the attenuation peak frequency can be adj...

Ceramics (Ba 1 − x Sr x)TiO 3 , (BST) with x=0.25; 0.35; 0.40; 0.50; 0.60; 0.75; 0.90 were prepared by sintering method from BaCO 3 , SrCO 3 and TiO 2 of high purity (99.98%). The solid-state reaction was employed to obtain pure and doped BST ceramic samples. In order to improve the sintering process the following additives were added: 1.0 wt.% MgO and 1 wt.% MnO 2. The sintering treatment was performed in the temperature range 1200 to 1260°C, for 2 h. Archimedean method was used for ceramic density measurements, which was found to depend linearly on strontium content: ρ(x)=5.4 2-0.8 8 •x, (g/cm 3). SEM, EDX and XRD methods were used for sample characterization. Morphology, pores and grain size distribution of both pure and doped ceramics were investigated by SEM analysis. A bimodal grain distribution could be observed in pure BST for x≤0.25, whereas a uniform one was noticed for doped samples of x=0.25. Well faceted grains of ∼5 μm size, could be seen for x=0.75 samples. Pores size has grown with the increase of Sr content. X-ray diffraction patterns have shown a dominant perovskite structure and some other minor phases. Dielectric permittivity and loss were investigated at 1 kHz, in the temperature range −150 to+ 150°C, at a pace of ∼2°C /min. Sharp transitions, with higher peaks values of permittivity were noticed with the increase of the sintering temperature. A shift of the Curie points to lower values was noticed with the increase of Sr content, according to the equation: Tc= 127.4−331•x. Measured at room temperature, in the frequency range of ∼1 GHz, permittivity was found to increase and losses to decrease with the sintering temperature. For x≤0.35 permittivity of the samples, being in the ferroelectric phase could not be measured in microwave domain at room temperature. Our results point to important application of these BST compositions for microwave devices.

We report on the temperature dependence, at microwave (mw) frequency, of the imaginary part of the dielectric constant (ε″) in myoglobin powder samples with different hydration levels (h). The measurements have been performed by the cavity perturbation technique, in the range of temperature 80–345 K. The sample is located inside a glass capillary along the axis of a cylindrical copper cavity, resonating in the TE011mode at 9.6 GHz, where the mw electric field has a node. By measuring the variation of the quality factor of the resonant cavity, one can extract the imaginary part of the dielectric constant. At temperatures higher than 230 K we observe an evident increase of the dielectric losses with increasing temperature; the effect scales almost linearly with hydration, indicating that it must be attributed to a relaxation of water in the hydration shell of the protein. Furthermore, ath≥0.18, we observe a clear peak in the ε″ vs.Tcurve, that shifts towards lower temperatures upon in...

Ceramic materials display a wide range of properties that facilitate their use in many different product areas. Currently, there has been keen interest in the field of ceramic materials due to their excellent mechanical and physical properties. Barium Stannate Titanate (BST) is a binary solid solution system composed of ferroelectric Barium titanate and non-ferroelectric barium titanate. In this study, the phase transition behavior of (Ba 1-x Sn x) TiO 3 (x = 0.5) (BST) ceramics was obtained by the dry-route method. The previous studies were based on Sn 2+ on the Ti site with varying values of x. The powders after calcination are compacted in the form of pellets using a hydraulic press at an optimized load above 70 KN. The experimental density of our sample measured by liquid displacement method with glycerin was lower than theoretical density, giving the shape is highly dense with low porosity. The structure shows that on increasing the Sn 2+ content volume decreases due to the size of Sn 2+ , which is smaller than that of Ba 2+ , in comparison to BaTiO 3. As the demand of lead-free environment-friendly sensor is increasing, thus obtained BST has great applications as a sensor material in modern electronic devices.

LiFe 5 O 8 (LFO) spinel-like material has been studied for use in ferrite resonator antennas (FRAs). Antenna parameters such as gain and return loss were greatly affected when an external magnetic field was applied to the FRA. The temperature coefficient of the resonant frequency (s f) for the FRA presented a value of À 482.16 ppm/°C. The magnetic hysteresis results showed that the LFO was a soft ferrite, considering the values of the remanent magnetization (M r = 5.95 emu g À1), coercive field (0.76 mT), and saturation magnetization (32.15 emu g À1). The magnetodielectric resonator presented a tuning effect in the resonant frequency as a function of the external magnetic field. The antenna bandwidth was also affected by the presence of the magnetic field. LFO is a soft ferrite with applications in microwave circuits, antennas, and devices for operation at microwave frequencies due to its magnetization and demagnetization properties. Impedance study revealed increasing conductivity from room to higher temperature with low activation energy (0.36 eV).

Bi 5 FeTi 3 O 15 (BFT) polycrystalline ceramic with the addition of different concentrations of V 2 O 5 was obtained by a solid-state method. X-ray powder diffraction, Raman spectroscopy and scanning electron microscopy (SEM) were used to study the microstructure and crystalline phases of the ceramics. SEM images showed plate-like morphology with dimensions between 0.32 lm and 3.07 lm (grain size, average around 1.3 lm). For samples with V 2 O 5 concentration below 5%, Raman spectra were mainly determined by the vibrational modes from BFT. Impedance spectroscopy was also performed to evaluate the dielectric properties at microwave and radio frequencies (RF). Two extra phases (Bi 4 V 1.5 Fe 0.5 O 10.5 and Bi 2 Ti 2 O 7) were found due to the chemical reaction between BFT and V 2 O 5. These phases were responsible for the changes in the grain morphology and dielectric response. V 2 O 5 addition increased the real part of the dielectric permittivity (e¢) and reduced the dielectric loss tangent (tan d) values at the RF range of 10 Hz to 1 MHz. For microwave frequencies of 3-3.5 GHz, e¢ and temperature coefficient of resonant frequency (s f) values ranged from 66.52 ppm/°C to 88.60 ppm/°C and À304.3 ppm/°C to À192.6 ppm/°C, respectively. Thereby, BFT ceramics with added V 2 O 5 are good candidates to be used for microwave devices (e.g., cell phones).

We report results on the structural and microwave properties and magnetic phase transitions in polycrystalline Sr3Co2Fe24O41 hexaferrite synthesized by sol-gel auto-combustion and acting as a filler in a composite microwave absorbing material. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves revealed a change in the magnetization behavior at 293 K. The reflection losses in the 1–20 GHz range of the Sr3Co2Fe24O41 powder dispersed homogeneously in a polymer matrix of silicon rubber were investigated in both the absence and presence of a magnetic field. In the latter case, a dramatic rise in the attenuation was observed. The microwave reflection losses reached the maximum value of 32.63 dB at 17.29 GHz in the Ku-band. The sensitivity of the microwave properties of the composite material to the external magnetic field was manifested by the appearance of new reflection losses maxima. At a fixed thickness tm of the composite, the attenuation peak frequency can be adj...

The objects of our research is magnesium aluminates (spinel), in which the electric charge of Al3+ cation is partially substituted with constitutive trivalent cation of Mg2+ and Ti 4+ in the system with the following composition: Mg1+xTixAl2-2xO4 with (x) varying from 0,1 to 0,6. X-ray diffraction data shows that within the above values of (x) the composition is a single phase spinel. The microwave basic parameters were measured as εr, tan δ and τf, respectively with the following values: εr = 6-12, tanδ = (1.5-3.5) x 10-4 at 10 GHz and Qf = 70 000 – 90 000, and τf from -50 to -70 ppm/deg.

Zr0.8Sn0.2TiO3 (ZST) powders synthesized by solid-state reaction were subject to processing by spark plasma sintering (SPS). A single-phase ceramic with a high relative density of 95.7% and 99.6% was obtained for sintering temperatures of 1150 °C and 1200 °C, respectively, and for a dwell time of 3 min. In order to reduce the oxygen vacancies, as-sintered discs were annealed in air at 1000 °C. The dielectric loss of the annealed samples, expressed by the Q × f product, measured in the microwave (MW) domain, varied between 35 THz and 50 THz. The intrinsic losses (Q × f ~ 60 THz) were derived by using terahertz time-domain spectroscopy (THz-TDS).

In this work, BBT has been synthesized by solid-state reaction. X-ray diffraction and Raman spectroscopy were used to structurally characterize the samples. The dielectric properties were analyzed by impedance spectroscopy for radiofrequency range. For microwave range, the dielectric properties were analyzed by the Hakki-Coleman method. The properties of a DRA made from BBT were obtained by numerical simulations and compared with experimental results. The results show that the synthesis of BBT was effective from 800 • C, showing nanosize particles (35-45 nm) with tetragonal symmetry. Raman spectra exhibited all active modes characteristic of BBT. The dielectric properties of BBT presented high values of dielectric permittivity (ε = 52.40 for microwave range and ε > 100 for radiofrequency range). BBT ceramic based showed negative values of (-422.11 ppm • C -1 ) temperature coefficient of resonant frequency. The DRA BBT-based demonstrated the use as an antenna, exhibiting small gain of 1.1 dBi and efficiency of 32.5%.

Abstract:- This paper deals with the Interference between Fixed Satellite Service Earth Station (FSS-ES) receiver and Fixed Wireless Access (FWA) in term of possible separation distance, for Malaysian environment. The study began with detailed clarification of worldwide regulatory and C-Band user history. Subsequently, detailed calculations of the current and most useful formulas for path loss effect and clutter loss by using the existing parameters of both services had been done. Thereafter, FSS axis receiving gain and radiation pattern covered in term of maximum possible separation reduction. Moreover, Site shielding, isolation, off-Axis, In-Band and out of band have been discussed in several scenarios. Calculations, analysis and simulation have been done by using Matlab software. Extracted conclusions highlighted the subsistent situation and proposals for future work. Key-Words:- Interference, FSS, FWA, Malaysia, LNB and Saturation. 1

This work presents a comparison between the dielectric properties of the SrBi2Nb2O9(SBN) ceramic matrix and the individual additions of Bi2O3 and La2O3, suggesting the material that would be most suitable for microwave applications. The SBN sample was synthesized by solid-state reaction, and the X-ray diffraction method (XRD) was used for the structural characterization of the materials analyzed in this study. Thermal stability of the samples was measured experimentally by studying the temperature coefficient of resonant frequency (τf ) the results confirm that the individual additions of 15 wt% of Bi2O3 or La2O3 improved the thermostability of the composites. The Hakki-Coleman method was used to obtain the dielectric properties in the microwave range, and it is possible to observe that the dielectric permittivity of the composites increased, while their loss tangents decreased. The numerical simulation demonstrated the functioning of the materials as dielectric resonating antennas ...

3D Finite Element Method simulations were employed in order to describe tunability properties in anisotropic porous paraelectric structures. The simulations predicted that properties of a ceramic can be tailored by using various levels of porosity. Porous Ba 0.6 Sr 0.4 TiO 3 (BST) ceramics have been studied in order to investigate the influence of porosity on their functional properties. The BST ceramics with various porosity levels have been obtained by solid-state reaction. Lamellar graphite in different concentration of 10, 20 and 35 vol.% was added as sacrificial pore forming agent. The structural, microstructural, dielectric and tunability properties were investigated. By comparison with dense BST ceramic, porous samples present a fracture mode transformation from intragranular to an intergranular fracture and a decrease of grain size. Lower dielectric constants, low dielectric losses, but higher values of tunability than in the dense material were obtained in the porous BST structures as a result of local field inhomogeneity generated by the presence of air pores-ceramic interfaces.

One of the main objectives of this work was to study the possibilities of valorising the phosphates through the development of a conductive ceramics using NPK fertilizer as a precursor. Phosphorus based powders were synthesized using solid state technique from NPK fertilizer, lithium chloride and iron chloride at different temperatures up to 900°C and ceramic samples were prepared by the powder pressing and sintering at 1100°C. XRD spectra of the calcined powders show various sharp peaks indicating a relatively high degree of crystallinity and presence of different crystalline phases, such as: phosphorus based crystalline compounds (AlPO 4 and LiFePO 4 ), ferrite (Fe 3 O 4 and DyFeO 3 ), CaSO 4 and K 3 DyCl 6 . The prepared phosphorus based ceramics showed very interesting electrical and dielectric properties. Thus, in the future the obtained ceramics could find application in electronic or energy storage devices. However, further investigations are necessary to understand the exact chemical composition and structural characteristics of this material, to better understand the origin of the obtained electrical and dielectric behaviour.

Ba 1Àx Sr x TiO 3 solid solutions were prepared by solid-state reaction from raw materials. Four compositions with x ¼ 0.25, 0.50, 0.75 and 0.90 have been investigated. The perovskite type and polycrystalline structure of the BST samples were revealed by X-ray diffraction data. The morphology, grain size distribution, porous structure and elemental composition of the sintered ceramics were analyzed by using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The temperature dependence of permittivity and of dielectric loss tangent at low frequency (1 kHz) showed decrease of Curie temperature with increase of Sr content. Microwave measurements (1 GHz) showed substantial decrease of the dielectric constant from about 1600 to 200 and also of the losses from 12% to less than 2% with the Sr concentration increase from x ¼ 0.25 to x ¼ 0.90. Moreover, the addition of MgO and MnO 2 1 wt.% each, improved sintering process and lowered the microwave losses up to 0.2%.

tIn this work, BBT has been synthesized by solid-state reaction. X-ray diffraction and Raman spectroscopywere used to structurally characterize the samples. The dielectric properties were analyzed by impedance spectroscopy for radiofrequency range. For microwave range, the dielectric properties were analyzedby the Hakki–Coleman method. The properties of a DRA made from BBT were obtained by numericalsimulations and compared with experimental results. The results show that the synthesis of BBT waseffective from 800◦C, showing nanosize particles (35–45 nm) with tetragonal symmetry. Raman spectraexhibited all active modes characteristic of BBT. The dielectric properties of BBT presented high values ofdielectric permittivity (ε'= 52.40 for microwave range and ε'> 100 for radiofrequency range). BBT ceramic based showed negative values of (−422.11 ppm◦C−1) temperature coefficient of resonant frequency. The DRA BBT-based demonstrated the use as an antenna, exhibiting small gain of 1.1 dBi and efficiency of 32.5%.

The influence of codoping of Fe, Co, Ni, Cu as acceptors and F as donor on the dielectric properties of screen-printed Ba 0.6 Sr 0.4 TiO 3 ceramic thickfilms has been investigated. The undoped and codoped Ba 0.6 Sr 0.4 TiO 3 powders were synthesized through a solgel route. The thermal gravimetric analysis revealed the difference of the thermal decomposition behaviour between the undoped and codoped precursors. The ceramic powders were characterized with x-ray diffraction, scanning electron microscopy and BET measurements. Larger crystallite sizes of the codoped powders were observed. The densification behaviour of the powders was recorded by dilatometry and indicated that codoping influences the sintering mechanism. The permittivity, dielectric loss and tunability of the undoped and codoped thick-films were characterized with coplanar waveguide structures up to 30 GHz.

Ba 0.6 Sr 0.4 (Ti 1Àx Mg x )O 3 (BSTM) ceramics were prepared by using a solid-state reaction process. X-ray diffraction results indicated that the solubility limit of Mg in Ba 0.6 Sr 0.4 TiO 3 was about 3 mol%. Doping of Mg into BST led to diffused cubictetragonal (C-T) phase transition and reduced Curie temperature. The diffused parameter g obtained by fitting the modified Curie-Weiss laws increased with increasing Mg content. Dielectric properties of the BSTM ceramics were measured. Figure of merit of BSTM3 was up to 127.8 at 17.5 kV/cm. Its dielectric loss was as low as 0.00133 at 1 kHz and room temperature.

Ba (1-x) Sr (x) TiO 3 ceramics, where x=0.05, 0.10, 0.15, and 0.20, were prepared by solid state reaction technique. X-ray diffraction analysis confirmed the formation of single phase perovskite structures. Mechanical properties such as ultrasonic attenuation, longitudinal wave velocity, and longitudinal elastic modulus were studied by an ultrasonic pulse echo technique at 2 MHz. Investigations of ceramic microstructures and mechanical properties showed its dependence on composition and sintering time. Increasing of Sr content, x, resulted in a decrease in bulk density and ultrasonic attenuation and an increase in velocity and modulus. Also, size of crystallites and c/a ratio were found to decrease with increasing of x, and increased with increasing of sintering time. High temperature ultrasonic studies showed, in addition to Curie phase transition, two relaxation peaks and its origin was investigated. The acquired structural and mechanical results were correlated and discussed.

Macroporous barium strontium titanate ceramics consisting of closed cells with an average size of approximately 10 μm and exhibiting porosities in the range of 65-80% were synthesized using particle-stabilized emulsions. These ceramics, after sintering at 1450 1C for 2 h, exhibited high crystallinity and a cubic perovskite structure. The synthesized high-porosity macroporous ceramics exhibited a significantly lower dielectric constant ( $ 315, when the porosity was 80%) than fully dense ceramics. The dielectric loss of the ceramics increased slightly with an increase in porosity, yet remained less than 0.2%.

Strontium bismuth titanate oxide (SrBi 4 Ti 4 O 15 ) belongs to the Aurivillius structural type. The materials in this family are good candidates as lead-free ferroelectric materials for applications at high temperature due to their elevated Curie temperature. The mechanochemical activation method permits one to reduce the particle size of the initial products, so the specific surface is increased, improving in most cases its reactivity. In this paper we report on the synthesis of SrBi 4 Ti 4 O 15 Aurivillius-type material from activated precursors, obtained though mechanical treatment in vibrating and planetary mills. In some samples a new transitory Sr-Bi-Ti-O fluorite-type phase appears, depending on the milling time and the kind of mill used. The synthesis of SrBi 4 Ti 4 O 15 from activated precursors is obtained at a temperature about 450 • C lower than it would have been required by a conventional solid-state reaction, with significantly reduced heating times. C 2004 Kluwer Academic Publishers 1. Introduction Lead-free Aurivillius phases [1] have been investigated in the last decade for their ferroelectric behaviour at high temperatures [2, 3] (Curie temperature in the range of 500 • C and upwards [4]) with important applications as ferroelectric materials [5]. The framework of the Aurivillius family consists of layers of Bi 2 O 2 where Bi 3+ can be partially substituted for by cations having a lone pair of electrons [6], intercalated with peroskite-type layers of the form A n−1 B n O 3n+1 , where n is the number of perovskite-like stacked layers [7], and where the voluminous cation is A [8-11], and B makes up the octahedra (BO 6 ) [12, 13], linking with Bi 2 O 2 blocks.

Ceramics (Ba1−xSrx)TiO3, (BST) with x=0.25; 0.35; 0.40; 0.50; 0.60; 0.75; 0.90 were prepared by sintering method from BaCO3, SrCO3 and TiO2 of high purity (99.98%). The solid-state reaction was employed to obtain pure and doped BST ceramic samples. In order to improve the sintering process the following additives were added: 1.0wt.%MgO and 1wt.%MnO2. The sintering treatment was performed in the temperature range 1200 to 1260 °C, for 2 h. Archimedean method was used for ceramic density measurements, which was found to depend linearly on strontium content: ρ(x)=5.42–0.88 ·x, (g/cm3). SEM, EDX and XRD methods were used for sample characterization. Morphology, pores and grain size distribution of both pure and doped ceramics were investigated by SEM analysis. A bimodal grain distribution could be observed in pure GST for x≤0.25, whereas a uniform one was noticed for doped samples of x=0.25. Well faceted grains of ∼5 μm size, could be seen for x=0.75 samples. Pores size has grown with the increase of Sr content. X-ray diffraction patterns have shown a dominant perovskite structure and some other minor phases. Dielectric permittivity and loss were investigated at 1 kHz, in the temperature range −150 to+150 °C, at a pace of∼2 °C /min. Sharp transitions,with higher peaks values of permittivity were noticed with the increase of the sintering temperature.Ashift of theCurie points to lower valueswas noticed with the increase of Sr content, according to the equation: Tc=127.4−331∙x. Measured at room temperature, in the frequency range of ∼1 GHz, permittivity was found to increase and losses to decrease with the sintering temperature. For x≤0.35 permittivity of the samples, being in the ferroelectric phase could not be measured in microwave domain at room temperature. Our results point to important application of these BST compositions for microwave devices.