BST Powders Research Articles

Low-temperature fabrication of NiCuZn Ferrite–(Ba<sub>0.7</sub>Sr<sub>0.3</sub>)TiO<sub>3</sub> composites using granulated powders

NiCuZn ferrite–(Ba0.7Sr0.3)TiO3 (BST) composite ceramics with the BST content of up to 50 vol % were fabricated by sintering compacted powder mixtures at 900°C. The granulation of the BST powder and addition of a Bi2O3-based oxide additive effectively assisted the densification of the composite samples. The samples thus fabricated showed required εr values whereas their μr values were considerably reduced, especially for those with lower BST contents. From the detailed examinations on the effect of the Bi2O3-based oxide additives and on the process modification to improve the reduced μr values, the addition of a Bi2O3–ZnO additive with the eutectic composition of as low as 1 wt % was enough to produce a densified 10 vol % BST composite sample with a highest μr value. It was also found that the incorporation of ferrite granules calcined at 600°C caused further improvement of μr. Thus, a densified 10BST composite sample with μr = 125 was fabricated by low-temperature sintering at 900°C.

Correlation of the microstructure and microwave properties of Ba0.6Sr0.4TiO3 thick-films

Abstract The influence of the microstructure of screen-printed Ba 0.6 Sr 0.4 TiO 3 ceramic thick-films on their microwave properties has been investigated. The Ba 0.6 Sr 0.4 TiO 3 powders were synthesized through a sol–gel route. The spray-dried precursors were calcined at different temperatures. Scanning electron microscopy images and BET measurements revealed the primary particle size of calcined powder depending on the calcination temperature. The Ba 0.6 Sr 0.4 TiO 3 thick-films showed different microstructure after sintering due to the use of two different calcined BST powders as well as varying the sintering conditions. Permittivity, dielectric loss and tunability of thick-films were characterized with coplanar waveguide structures up to 40 GHz. A correlation of microstructure and microwave properties of the thick-films has been observed. Particularly the tunability could be improved by an optimized microstructure.

Synthesis and Characteristics of MgO Coated BST-Mg2TiO4 Composite Ceramics

The (Ba, Sr)TiO3 (hereafter BST) ceramics are promising candidate for applying in tunable devices. MgO coated BST-Mg2TiO4 (BSTM-MT) composite ceramics were prepared to obtain the low dielectric constant, low dielectric loss, good dielectric constant temperature stability, and high tunability of BST ceramics. The Ba0.55Sr0.40Ca0.05TiO3 nanoparticles were coated with MgO using the precipitation method and then mixed with Mg2TiO4 powders to fabricate BSTM-MT composite ceramics. The morphologies, phases, elements, and dielectric properties of the sintered ceramics were investigated. The core-shell structure of BST powder wrapped with MgO was clearly observed from the TEM image. After sintered at 1100 °C for 2 h, the composite ceramics expressed dense microstructures from SEM images and two main phases BST and Mg2TiO4 were detected in the XRD patterns. The dielectric constant and loss tangent were both reduced after the coating. The reduced dielectric constant and loss tangent of BSTM-MT were 190, 0.0011 (2MHz), respectively. The ceramics exhibited the diffuse phase transition near the Curie temperature and the Curie temperature shifted from 10 °C to 5 °C after the coating. Since the continuous Ti-O bonds were disconnected with the MgO coating, the tunability was reduced to 15.14 % under a DC bias field of 1.1 kV/mm. The optimistic dielectric properties made it useful for the application of tunable capacitors and phase shifters.

The Effect of B<sub>2</sub>O<sub>3</sub>–Li<sub>2</sub>CO<sub>3</sub> Addition on Sintering Behavior and Dielectric Properties of Ba<sub> 0.6</sub>Sr<sub>0.4</sub>TiO<sub>3</sub> Ceramics

Barium strontium titanate (Ba0.6Sr0.4TiO3: BST) powders were prepared by the oxalate co-precipitation method. The ceramics from thus obtained BST powders with B2O3-Li2CO3 addition were prepared by conventional oxide mixing method. The powders and the ceramics were observed and analyzed by SEM and X-ray diffraction. The effects of B2O3-Li2CO3 addition and powder properties on the sintering behavior and the dielectric properties of BST-based ceramics were investigated. SEM results revealed that the BST powders are micron-sized with cauliflower-shaped rough surface leading the specific surface area reaches 18.52m2/g. In dilatometric studies, the ceramics without B2O3-Li2CO3 addition didn’t produce the desired shrinkage and dense microstructure at relative low temperatures (<1000°C). However, the sintering temperature is decreased to 850°C by addition of small amount (≤3wt %) of B2O3 and Li2CO3. This was also verified in sintered microstructures. The XRD results showed that the main phase of the ceramics was BST without any other crystalline phases of remarkable extent. With increasing of B2O3-Li2CO3 content, the permittivity and the tunability of the ceramics were decreased.

Synthesis of fine-crystalline Ba 0.6Sr 0.4TiO 3–MgO ceramics by novel hybrid processing route

We report the use of a novel powder-in-sol precursor hybrid processing route to synthesize dense, homogeneous, and fine-crystalline Ba 0.6Sr 0.4TiO 3–MgO (BST–MgO) ceramics as well as the study of the sintering behavior, microstructures, and dielectric properties of the ceramics. Nanosized BST powders are dispersed into BST sol–gel precursor and uniformly distributed BST slurry is obtained after ball-milling mixing. Mg(NO 3)·6H 2O solution is added to the BST slurry to give homogeneous BST–MgO slurry upon ball-milling mixing. The BST–MgO slurry is dried and calcined prior to pressing and sintering at low temperatures of 1200–1300 °C to form the ceramics. The ceramics possess very low dielectric loss tangent below 0.005 for frequency above 1 kHz and for temperature in the range −190–80 °C. The dielectric constant and dielectric tunability increase, while the ferroelectric transition broadening decreases, with increasing average grain size.

Effect of dispersant on preparation of barium–strontium titanate powders through oxalate co-precipitation method

The quantitative precipitation of barium–strontium titanyl oxalate: (Ba 0.6Sr 0.4TiO(C 2O 4) 2·4H 2O, BSTO) precursor powders were successfully prepared through oxalate co-precipitation method. The pyrolysis of BSTO at 800 °C/4 h produced the barium–strontium titanate (Ba 0.6Sr 0.4TiO 3, BST) powders. Two kinds of dispersants namely ammonium salt of poly mathacrylic acid (PMAA-NH 4) and polyethylene glycol (PEG) were added respectively during the co-precipitation procedure. The powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), etc. Experimental results show that the addition of the dispersants reduced the productive rate of BST powders. The BSTO and BST powders obtained by aforementioned technique without dispersants were homogeneous with quasi-orbicular morphology. The particles grew into spindle shape with the effect of PEG. The morphology homogeneity was broke with small grains as well as large agglomerated particles concurrent when PMAA-NH 4 was added. The mechanism of the effect of the two dispersants was investigated in detail.

Influence of Bi 2O 3 and CuO addition on low-temperature sintering and dielectric properties of Ba 0.6Sr 0.4TiO 3 ceramics

In this study, we tried to lower the sintering temperature of Ba 0.6Sr 0.4TiO 3 (BST) ceramics by several kinds of adding methods of Bi 2O 3, CuO and CuBi 2O 4 additives. The effects of different adding methods on the microstructures and the dielectric properties of BST ceramics have been studied. In the all additive systems, the single addition of CuBi 2O 4 was the most effective way for lowering the sintering temperature of BST. When CuBi 2O 4 of 0.6 mol% was mixed with starting BST powders and sintered at 1100 °C, the derived ceramics demonstrated dense microstructure with a low dielectric constant ( ɛ = 4240), low dielectric loss (tan δ = 0.0058), high tunability (Tun = 38.3%) and high Q value ( Q = 251). It was noteworthy that the sintering temperature was significantly lowered by 350 °C compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST.

Preparation of Barium Strontium Titanate Ceramics via Combustion Method

This study concentrated on the crystal structure and microstructure of [(Ba0.75Sr0.25)TiO3; (BST)] ceramics at different firing temperatures. The BST powders were prepared by a combustion method. (CO(NH2)2) was used as a fuel. Crystallinity of the calcined powders was improved by increasing the calcining temperature, as indicated by the increase in intensity of the X-ray diffraction peak. The pure perovskite phase of BST powders was obtained with a calcinations condition of 1200 oC. The a axis lattice constant of BST calcined powders and sintered ceramics were calculated and it was found that the crystal structure is a cubic phase. The microstructure of BST calcined powders and sintered ceramics were analyzed by a scanning electron microscope (SEM). The SEM result indicated that the average particle size and average grain size increased with the increase of calcinations and sintering temperatures, respectively. The apparent density of the samples was measured by the Archimedes method.

Preparation of cauliflower-like shaped Ba 0.6Sr 0.4TiO 3 powders by modified oxalate co-precipitation method

The quantitative barium–strontium titanyl oxalate (Ba 0.6Sr 0.4TiO(C 2O 4) 2·4H 2O, BSTO) precursor powders were prepared by the modified oxalate co-preparation method. It was based on the cation-exchange reaction between the stoichiometric solutions of oxalotitanic acid (H 2TiO(C 2O 4) 2, HTO) and barium + strontium nitrate solution containing stoichiometric quantities of Ba and Sr ions. The pyrolysis of BSTO at 800 °C/4 h in air produced the homogeneous cauliflower-like shaped barium–strontium titanate (Ba 0.6Sr 0.4TiO 3, BST) powders. The effect of polyethylene glycol (PEG) on morphology of BSTO and BST powders was also investigated. The characterization studies were carried on the as-dried BSTO and calcined BST powders by various physicochemical techniques, IR, X-ray diffraction (XRD), scanning electron microscopy (SEM), etc. The BSTO and BST powders obtained by aforementioned technique without PEG were homogeneous with spherical shape. The particles grew into spindle shape with the effect of PEG.

Simple oxalate precursor route for the preparation of brain-like shaped barium–strontium titanate: Ba 0.6Sr 0.4TiO 3

Through adding quantitative ammonia into a precursor solution containing stoichiometric quantities of Ba and Sr ions before the co-precipitation procedure, a simple oxalate co-precipitation method based one-step cation-exchange reaction between the stoichiometric solutions of oxalotitanic acid (HTO) and barium + strontium nitrate is investigated successfully for the quantitative precipitation of barium–strontium titanyl oxalate (BSTO): Ba 0.6Sr 0.4TiO(C 2O 4) 2·4H 2O precursor powders. The pyrolysis of BSTO at 800 °C/4 h in air produced the homogeneous brain-like shaped barium–strontium titanate (Ba 0.6Sr 0.4TiO 3: BST) powders. The characterization studies were carried on the as-dried BSTO and calcined BST powders by various physicochemical techniques, IR, DSC/TGA, XRD, SEM, etc. It revealed that the BST powders are cubic, stoichiometric, highly pure, sub-micron-sized with nearly uniform size, brain-like shape and agglomerated nature.

Structural and electric properties of barium strontium titanate based ceramic composite as a humidity sensor

Ba 0.6Sr 0.4TiO 3–MgTiO 3 (BST–MT) composite ceramics were prepared by reaction of BST powders with molten MgCl 2. The sintered composites exhibit a five orders of magnitude decrease in the dc resistivity as the relative humidity (RH) was increased from 5 to 92%. The ac conductivity of the samples obeys a power law at different temperatures within the frequency range of 10 Hz to 10 MHz.

Synthesis of nanosized perovskite (Ba,Sr)TiO 3 powder via a PVA modified sol-precipitation process

Ba 0.55Sr 0.45TiO 3 (BST) precursors were synthesized via a polyvinyl alcohol (PVA) modified sol-precipitation route. And the obtained precursors were then calcined in air at temperatures ranging from 400 to 800 °C. The formation mechanism of BST phase was investigated using X-ray diffraction (XRD), while the effect of PVA on the particle morphology of the BST phase was studied by using scanning electron microscopy (SEM). The results show that the introduction of PVA significantly affects the morphology of the BST powders. High purity nanocrystalline powders were synthesized by calcination of the BST precursors encapsulated by PVA gel, with narrow particle distribution (30–80 nm) and being nearly free of agglomeration. While, large particles (40–170 nm) with evident agglomeration were obtained from the solution without PVA.

Microstructure and dielectric properties of ferroelectric barium strontium titanate ceramics prepared by hydrothermal method

Ba x Sr1-x TiO3, nanoparticles with different Ba compositions were synthesized by a hydrothermal method. The mechanism of hydrothermal reactions was discussed based on DTA/TG, XRD and TEM characterizations. The result showed that perovskite structure was developed through the mutual diffusion between the intermediate phases and TiO2 phase. The grain size of the Ba0.77Sr0.23TiO3 (BST77) powders was about 20–40 nm. BST ceramics were made from the hydrothermal-derived BST powders and the dielectric properties of the BST ceramics were measured. Due to the small grain size and active surface energy of the BST powders prepared by hydrothermal method, the BST ceramics showed low sintering temperature. It was found that the BST77 ceramics sintered at 1280 °C showed dielectric constant peak dispersion which was believed to be caused by dimension domino effect.

MgTiO3 doping effect on dielectric properties of Ba0.6Sr0.4TiO3 ceramics via a molten salt process

Ba0.6Sr0.4TiO3–MgTiO3 composites were prepared by reaction of BST powders with molten MgCl2 at various MgCl2/BST molar ratios at 600 and 800 °C in air. The products were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is shown that the ceramic composites have a relaxor-like ferroelectric transition as MgTiO3 is above 8 mol%. The addition of MgTiO3 has greatly reduced the dielectric loss of BST ceramics. The BST–MT ceramics could be a promising candidate for tunable microwave device applications.

Low temperature synthesis of Ba 0.70Sr 0.30TiO 3 powders by the molten-salt method

Pure perovskite phase Ba 0.70Sr 0.30TiO 3 (BST) powders were successfully synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 850 °C for 2 h, which is 300 °C lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcining of the mixtures led to BST powders at 800–900 °C. XRD and SEM techniques are used to characterize the phase and morphology of the fabricated BST powders, respectively.

Morphologies and crystal structures of nano-sized Ba 1− xSr xTiO 3 primary particles prepared by flame spray pyrolysis

Nano-sized Ba 1− x Sr x TiO 3 (BST) powder was prepared by flame spray pyrolysis using “CA-assisted” spray solution. The effects of the mole ratios of Ba to Sr components on the mean sizes, morphologies, and crystal structures of the BST powder prepared by flame spray pyrolysis were investigated. The precursor powders obtained by flame spray pyrolysis had large size, fractured and hollow structures irrespective of the mole ratios of Ba to Sr components. The post-treated BST powders had slightly aggregated morphology of the primary particles with nanometer sizes. The slightly aggregated BST powders turned to nano-sized primary particles by a simple milling process. The milled BaTiO 3 particles post-treated at temperature of 1000 °C had spherical-like shape. On the other hand, the milled Ba 0.5Sr 0.5TiO 3 and SrTiO 3 particles had square shape. The mean sizes of the milled BaTiO 3, Ba 0.5Sr 0.5TiO 3 and SrTiO 3 particles were each 110, 32, and 48 nm. Phase pure BST powder was obtained at a post-treatment temperature of 1000 °C irrespective of the mole ratios of Ba to Sr components. The BaTiO 3 powder had tetragonal crystal structure. On the other hand, the BST except for the BaTiO 3 composition had cubic crystal structures at post-treatment temperature of 1000 °C. The mean crystallite sizes of the milled Ba 1− x Sr x TiO 3 primary particles were changed from 29 to 37 nm according to the mole ratios of Ba to Sr components.

Microstructure and Dielectric Properties of Barium Strontium Titanate Thick Films and Ceramics With a Concrete‐Like Structure

Barium strontium titanate (Ba0.6Sr0.4TiO3, BST) thick films and ceramics with a concrete‐like structure were fabricated by a novel sol–gel‐based route. In this process, well‐crystallized nano‐sized or micrometer‐sized BST powders or their combinations are dispersed into a sol–gel matrix. Crack‐free thick films and dense BST ceramics have been fabricated at a low sintering temperature with a very inhomogeneous concrete‐like structure. The dielectric properties of such thick films and ceramics have been investigated. The results show that the dielectric constant and loss tangent of the thick films are 1209 and 0.016, respectively, at 0°C, 10 kHz, and those of the bulk BST ceramics are 5510 and 0.02, respectively, at 0°C, 10 kHz.

(Ba 0.5Sr 0.5)TiO 3 modification on etched aluminum foil for electrolytic capacitor

A new method was proposed to form (Ba 0.5Sr 0.5)TiO 3–Al 2O 3 composite oxide film on etched aluminum foils. The specimens were covered with (Ba 0.5Sr 0.5)TiO 3 (BST) layer by dip-coating in citrate solution and subsequent heat-treatment under 400–650 °C, finally by anodizing in a hot boracic acid and borate solution. The BST powders heated under different temperatures were characterized by X-ray diffraction (XRD) and the specific capacitance of the coated specimens heat-treated under different temperatures and times was measured. It is found that the specific capacitance increases initially with enhancing the temperature and reaches to maximum at 550 °C, but slightly decreases with the heat-treatment time. The capacitance was increased by about 35% after BST coating.

Structural and dielectric properties of barium strontium titanate produced by high temperature hydrothermal method

The preparation procedure, structural and dielectric properties of hydrothermally derived Ba x Sr 1− x TiO 3 (BST) were studied. BST with initial Ba compositions of 75, 80, 85 and 90 mol.% were prepared by a high temperature hydrothermal synthesis. The obtained powders were pressed into pellet, cold isostatically pressed and sintered at 1200 °C for 3 hours. The phase compositions and lattice parameters of the as prepared powders and sintered samples were analysed using X-ray diffractometry. A fitting software was used to analyse the XRD spectra to separate different phases. It was found that BST powder produced by the high temperature hydrothermal possessed a two-phase structure. This structure became more homogeneous during sintering due to interdiffusion but a small amount of minor phase can still be traced. Samples underwent an abnormal grain growth, whereby some grains grow faster than the other due to the presence of two-phase structure. The grain size increased with increasing Ba amount. Dielectric constant and polarisation increased with increasing Ba content but it was also affected by the electronic state and grain size of the compositions.

Formation and control of mechanism for the preparation of ultra-fine barium strontium titanate powders by the citrate precursor method

A modified citrate precursor method, combining the advantages of the nitrate autocombustion method by introducing the nitric acid, was successfully used to synthesize ultrafine Ba 0.70Sr 0.30TiO 3 powders. Slight agglomerated and homogenized BST powders, with an average particle size of 20 nm, were obtained at 800 °C. Combining the results of TG/DTA and XRD, it can be concluded that the formation of BST powders takes place via two reaction mechanisms: decomposition of an intermediate oxycarbonate mechanism (DIOm) at low temperature, and solid-state reaction mechanism (SSRm) between nanocrystalline carbonates and amorphous TiO 2 at high temperature. TEM shows higher amount of CA led to BST powders of better quality in morphology. Based on results of chemical analysis, it is suggested that higher amount of CA drives the mechanism to tend to DIOm. By simply adjusting the ratio of reagents, the reaction mechanism can be dominated and we can greatly control the final morphology of the powders.