Structural refinement,optical and microwave dielectric properties of BaZrO3

In this work, barium zirconate (BaZrO₃) ceramics synthesized by solid state reaction method and sintered at 1670 °C for 4 h were characterized by X-ray diffraction (XRD), Rietveld refinement, and Fourier transform infrared (FT-IR) spectroscopy. XRD patterns, Rietveld refinement data and FT-IR spectra which confirmed that BaZrO₃ ceramics have a perovskite-type cubic structure. Optical properties were investigated by ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) measurements. UV–vis absorption spectra suggested an indirect allowed transition with the existence of intermediary energy levels within the band gap. Intense visible green PL emission was observed in BaZrO₃ ceramics upon excitation with a 350 nm wavelength. This behavior is due to a majority of deep defects within the band gap caused by symmetry breaking in octahedral [ZrO₆] clusters in the lattice. The microwave dielectric constant and quality factor were measured using the method proposed by Hakki–Coleman. The dielectric resonator antenna (DRA) was investigated experimentally and numerically using a monopole antenna through an infinite ground plane and Ansoft's high frequency structure simulator software, respectively. The required resonance frequency and bandwidth of DRA were investigated by adjusting the dimension of the same material.     

Confocal fluorescence microscopy in alumina-based ceramics: Where does the signal come from?

Confocal Cr³⁺ fluorescence microscopy is an ideal technique for investigating residual stresses in alumina-based ceramics. Due to their transparency, however, it is important to understand where the collected signal comes from by characterising the probe response function (PRF). Here, a PRF is proposed that captures all the relevant physical effects, including a newly identified consequence of scattering by pores and grain boundaries. The new PRF describes the response of a range of alumina-based ceramics to depth scanning in a high resolution confocal fluorescence microscope in a manner that balances physical significance with the accuracy of empirical fitting. The results showed that measurements could be made deep within single crystals of sapphire and ruby, although refraction degraded the depth resolution from about 3 μm at the surface to 25 μm at a depth of 500 μm. Scattering and absorption limited the depth to which polycrystalline alumina could be probed to ～15 μm. This was further reduced to ～4 μm for an alumina–10 vol.% SiC nanocomposite. However, the absorption increased the accuracy of near surface measurements in these materials by preventing contamination from subsurface fluorescence.     

A new approach to producing transparent ZnAl2O4 ceramics

A new approach to producing of transparent bulk ZnAl₂O₄ ceramics based on hot pressing of powders (1600 °С, 50 МPа) in presence of sintering additive ZnF₂ is described. Using this approach in the presence of 5 wt% of ZnF₂ transparent ZnAl₂O₄ ceramics was prepared with transparency range from 0.2 to 7.5 μm and with band gap of about 6.05 eV. The average grain size was about 33 μm and the transmittance at the wavelength of 550 nm was about 63%.     

SEM investigation of domain structure in (Ba,Ca,Pb)TiO3

In the present paper the microstructure and domain structure in modified BaTiO₃ with Pb and Ca as additives have been investigated using SEM technique. The (Ba,Pb)TiO₃ and (Ba,Ca,Pb)TiO₃ ceramics show a slight difference in grain size, being smaller in composites with Ca additives which acts as grain growth inhibitor. The domain configuration is almost the same. The small grain microstructure with tiny domains have been observed in specimen sintered at 1300°C and the average grain size is in the range 1–3 μm. For those specimens sintered at 1320°C the homogenous microstructure is also obtained with grain size around 2–4 μm. For both types of specimens, the single domain structure is associated with grain which size is lower than 2 μm. The banded domain structure could be observed in grains with size bigger than 3 μm. The bar shape grains and elongated grains together with some large region in microstructure are free of domain structure. The observed domain patterns reveal mainly the straight domain boundary lines with 90° domains walls. The wall thickness ranged from 0·03 μm to 0·15 μm, while the domain width is in the range of 0·1 μm–1 μm.     

A novel multi−wavelength photoacoustic spectrometer for the measurement of the UV–vis-NIR spectral absorption coefficient of atmospheric aerosols

A multi-wavelength photoacoustic instrument is described, which measures the wavelength dependent optical absorption coefficient (OAC) of soot or soot-containing aerosols in-situ in a range from the ultra-violet to the near-infrared region. The instrument combines a Nd:YAG disc laser (fundamental wavelength 1064 nm, harmonics at 532, 355 and 266 nm) and four photoacoustic detection cells, each purged with the same aerosol sample flow, while being irradiated with one of the four light beams. With the help of a supplementary optical arrangement to illuminate each detection cell with 532 nm light, the system is calibrated against OAC by purging the cells with known concentrations (and hereby known OAC values) of NO₂. This calibration eliminates differences in sensitivity of the detection PA cells and makes the measurement of OAC absolute.The minimum detectable OAC was determined to be 0.2 Mm^?1 at 1064 nm and 35.5 Mm^?1 at 266 nm, corresponding to a minimum detectable black carbon mass concentration of about 0.1–1 μg/m³, depending on the wavelength. Comparison measurements with artificially generated soot aerosols showed good agreement of the device with a reference instrument, based on a long path extinction cell (LOPES).     

The ZrC–C eutectic structure and melting behaviour: A high-temperature radiance spectroscopy study

A fast spectro-pyrometer has been employed for radiance measurements of zirconium carbide samples laser-heated to very high temperature, for compositions 0.7 ≤ C/Zr ≤ 2.61 and in a spectral range 0.550 μm ≤ λ ≤ 0.900 μm. The ZrC–C eutectic temperature has been taken as the radiance reference. The measured normal spectral emissivity (NSE) ?_λ of solid zirconium carbide is close to 0.6 at 0.650 μm, in agreement with previous literature. Its high-temperature behaviour, value in the liquid, carbon-content and wavelength dependences in the visible-near infrared range have been determined here for the first time. Liquid zirconium carbide seems to interact with electromagnetic radiation in a more metallic way than the solid. A considerable NSE increase has been observed at increasing carbon content, which can be interpreted on the basis of preferential growth along the “c” plane of the carbon lamellae in the eutectic structure.     

Multilayer 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 elements for electrocaloric cooling

Electrocaloric (EC) cooling elements in the form of multilayers (MLs) were prepared. The elements consist of five layers of the relaxor-ferroelectric 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃, about 60 μm thick, with internal platinum electrodes and exhibiting a dense, uniform microstructure with a grain size of 1.7 μm. The largest temperature change ΔT_EC of 2.26 K was achieved at an electric field (E) of 100 kV cm⁻¹ and at 105 °C, measured by a high-resolution calorimeter. These results agree well with the indirect measurements. The EC coefficient, ΔT_EC/ΔE, obtained for the MLs, is similar to the value obtained for bulk ceramics of the same composition. The ΔT_EC values above 2 K over a broad temperature range from 75 to 105 °C make the ML elements suitable candidates for EC cooling devices at significantly lower voltages than bulk ceramic plates with comparable dimensions and mass.     

Phenolic resin-based composite sheets filled with mixtures of reduced graphene oxide, γ-Fe2O3 and carbon fibers for excellent electromagnetic interference shielding in the X-band

Composite sheets consisting of phenolic resin filled with a mixture of reduced graphene oxide (RGO), γ-Fe₂O₃ and carbon fibers have been produced by compression molding. Its electrical conductivity lies in the range 0.48–171.21 S/cm. Transmission and scanning electron microscopy observations confirm the presence of nano particles of γ-Fe₂O₃ (～9.8 nm) and carbon fiber (～1 mm) which gives flexural strength to composite sheets. Thermogravimetric analysis show that the thermal stability of the sheets depend upon the amount of RGO and phenol resin in the composite. Complex parameters, i.e., permittivity (ε* = ε′ ? iε″) and permeability (μ* = μ′ ? iμ″) of RGO/γ-Fe₂O₃/carbon fiber have been calculated from experimental scattering parameters (S₁₁ and S₂₁) using theoretical calculations given in Nicholson?Ross and Weir algorithms. The microwave absorption properties of the sheets have been studied in the 8.2–12.4 GHz (X-Band) frequency range. The maximum shielding effectiveness observed is 45.26 dB, which strongly depends on dielectric loss and volume fraction of γ-Fe₂O₃ in RGO matrix.     

Proton conducting solid oxide fuel cells with chemically stable BaZr0.75Y0.2Pr0.05O3-δ electrolyte

A BaZrO₃-based electrolyte with low Pr-doping concentration is proposed as electrolyte for proton-conducting solid oxide fuel cells (SOFCs). The new material BaZr_0.75Y_0.2Pr_0.05O_3-δ (BZYP5) shows a good chemical stability against CO₂. In addition, the low doping concentration of Pr in BaZrO₃ improves the sinterability of BaZrO₃ and also allows its structure to remain stable even in the reducing atmosphere, which is critical for fuel cell applications. The cell with BZYP5 as electrolyte shows maximum power densities of 124, 70, and 43 mW cm⁻² at 600, 550, and 500 °C, respectively, which are larger than that for the cell with conventional high Pr-doping BaZrO₃ electrolyte reported previously. Electrochemical analysis indicates that the BZYP5 electrolyte shows a good ionic conductivity. These results suggest that the low Pr-doping strategy presented in this study promotes the densification for BaZrO₃ and the good electrolyte conductivity of BaZrO₃ is maintained which could be the reason for the improved cell performance, suggesting BZYP5 is a promising electrolyte for proton-conducting SOFCs.     

Notable grain-size dependence of converse piezoelectric effect in BaTiO3 ceramics

Converse piezoelectric effect is of critical importance to device applications like actuators, but no systematical investigation concerning the influence of microstructure on it has been reported for BaTiO₃ ceramics so far. Piezoelectric and ferroelectric properties were inclusively investigated for a group of BaTiO₃ ceramics that are fabricated through solid-state reaction route and show various average grain sizes in this study. It was found that the piezoelectric properties of these BaTiO₃ ceramics display significant grain-size dependences. The direct piezoelectric coefficient d₃₃ increases with decreasing the average grain size (GS) from 170 pC/N at 40 μm, reaches a maximum value of 413 pC/N at 1.2 μm, and then decreases with a further reduction of GS. Converse piezoelectric effect was characterized by measurement of unipolar strain versus electric field (S–E) curve, and the converse piezoelectric coefficient d₃₃*(E) was quantitatively calculated from the slope of S–E curve at relatively large E. Interestingly, d₃₃*(E) is nearly twice as large as d₃₃ and shows a quite similar trend of change with GS to d₃₃. It increases largely from 350 pm/V to 870 pm/V when reducing the GS value from 40 μm to 1.2 μm, and then decreases to 480 pm/V with the further GS reduction to 0.7 μm. Meanwhile, the remanent polarization P_r shows an increase with the decreasing of GS, reaches a maximum at 3.3 μm, and then decreases with the further GS reduction. Domain structure is considered to play an essential role in determining the notable grain-size dependence of converse piezoelectric effect.     

Advanced spinel and sub-μm Al2O3 for transparent armour applications

Hardness is important for a high ballistic strength, and with HV10 = 20–22 GPa sintered sub-μm Al₂O₃ is the hardest of all transparent materials for compact windows. However, light transmission through polycrystalline Al₂O₃ is limited by birefringent scattering losses: high transmissions are known at larger IR wavelengths for grain sizes of about 0.5 μm but the visible real in-line transmission RIT is only 70–75% of the theoretical maximum at 0.8–1 mm thickness. These losses will be the higher for thicker components whereas a safe ballistic performance requires 1.5–2 mm thickness at least. New technologies bring the transmission closer to the limit associating grain sizes of 0.3 μm with an RIT of 84–93% of the theoretical maximum (thickness 0.8 mm). However, even these extreme results give again rise to doubt that it will ever be possible to manufacture larger and thicker Al₂O₃ windows with a sufficiently high transparency.On the other hand, new results are presented for fine-grained spinel with RIT close to the theoretical maximum and with a hardness that approaches sapphire. In first ballistic tests this spinel outperformed sapphire of different orientations. It is, therefore, suggested that sub-μm Al₂O₃ may be a good choice for IR windows or as armour for low threat applications where thinner tiles can be used. Most threats, however, require thicker windows where the new spinel appears as one of the most favourable candidates.     

Grain size effect on electrical properties of Mn-modified 0.67BiFeO3–0.33BaTiO3 lead-free piezoelectric ceramics

To investigate how grain size affects the dielectric, ferroelectric, and piezoelectric properties of Mn-modified 0.67BiFeO₃–0.33BaTiO₃ ceramics, we prepared samples with a wide variety of grain sizes from 4.1 μm to 0.59 μm via a conventional solid-state process that use the normal and the two-step sintering methods. Small-signal dielectric measurements show that all the samples exhibit a relaxor-like behavior and that grain size has little influence on the room-temperature dielectric permittivity. For grain sizes below 2 μm, the remanent polarization P_r and piezoelectric coefficient d₃₃ decrease with the grain size, whereas they remain almost constant near P_r = 27 μC/cm² and d₃₃ = 70 pC/N in samples with grain sizes exceeding 2 μm. The mechanism underlying the observed grain size effect is discussed in terms of the electric-field-induced formation of macroscopic ferroelectric domains.     

Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics

A series of transparent ceramics with the composition of La_2−xLu_xZr₂O₇ (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La_0.8Lu_1.2Zr₂O₇ ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.     

Highly transparent lutetium titanium oxide produced by spark plasma sintering

Transparent Lu₂Ti₂O₇ pyrochlore was fabricated by reactive sintering using spark plasma sintering at 1723 K for 45 min. The sintered body exhibited 72% transmittance at a wavelength of 2000 nm and 40% transmittance at 550 nm. The average grain size was 14.5 μm with uniform microstructure.     

Yb3+ doped Lu2O3 transparent ceramics by spark plasma sintering

Transparent ceramics of 10% Yb doped Lu₂O₃ was fabricated by spark plasma sintering. The operating vital parameters in yielding transparency and mutual effects of sintering, pressure, dwell time, heating rate and annealing temperature on microstructure have been investigated. Fully compacted specimens were obtained at 1250 °C and the average grain size increased from few nm up to 5 μm until 1700 °C, above which abnormal grain growth was witnessed. The post-annealing of sintered ceramics at 1200 °C removes discoloration and improves transparency. The ceramics prepared at 1700 °C with dwell time of 5 min and heating rate at 50 °C/min shows the maximum transmittance with a thickness of 2 mm of 55% at a wavelength of 2 μm.     

Hydrothermal synthesis of dumbbell-shaped ZnO microstructures

Dumbbell-shaped ZnO microstructures have been successfully synthesized by a facile hydrothermal method using only Zn(NO₃)₂·6H₂O and NH₃·H₂O as raw materials at 150 °C for 10 h. The results from X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) show that the prepared ZnO samples exhibit dumbbell-shaped morphology and hexagonal wurtzite structure. The length of ZnO dumbbells is about 5–20 μm, the diameters of the two ends and the middle part are about 1–5 μm and 0.5–3 μm, respectively. The dumbbell-shaped ZnO microstructures may be formed by self-assembly of ZnO nanorods with 1–5 μm in length and 100–200 nm in diameter. The photoluminescence (PL) spectrum of dumbbell-shaped ZnO microstructures at room temperature shows three emission peaks at about 362, 384 and 485 nm.     

Structures and microwave dielectric properties of Ca(1−x)Nd2x/3TiO3 ceramics

Ca_(1?x)Nd_2x/3TiO₃ microwave dielectric ceramics were prepared by the mixed oxide route; powders were calcined at 1100 °C and sintered at 1450–1500 °C. High density, single phase products were obtained for all compositions. Grain sizes ranged from 1 μm to 100 μm. There was evidence of significant discontinuous grain growth in mid range compositions; all ceramics were characterised by complex domain structures. With increasing Nd content there was a evidence of a transition from an orthorhombic Pbnm structure to a monoclinic C2/m structure. This was accompanied by a decrease in relative permittivity (?_r) from 180 to 78, and decrease in the temperature coefficient of resonant frequency (τ_f) from +770 ppm K^?1 to +200 ppm K^?1. The product of dielectric Q value and resonant frequency (Q × f) varied in a grossly non-systematic way, exhibiting a peak at 13,000 GHz in Ca_0.7Nd_0.2TiO₃.     

Microwave dielectric properties of the (BaxSr1−x)TiO3 thin films on alumina substrate

Barium strontium titanate, (Ba_xSr_1?x)TiO₃ (BST) thin films have been prepared on alumina substrate by sol–gel technique. The X-ray patterns analysis indicated that the thin films are perovskite and polycrystalline structure. The interdigital electrode with 140 nm thickness Au/Ti was fabricated on the film with the finger length of 80 μm, width of 10 μm and gaps of 5 μm. The temperature dependence of dielectric constant of the BST thin films in the range from ?50 °C to 50 °C was measured at 1 MHz. The dielectric properties of the BST thin films were measured by HP 8510C vector network analyzer from 50 MHz to 20 GHz.     

Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells

Nanocrystalline SOFC cathode materials of perovskite family, La_1?xSr_xM_1?yCo_yO₃, where 0 < x ≤ 0.5, 0 < y ≤ 0.8 (M is transitional metal = Mn or Fe), have been synthesized at a relatively low temperature by combustion synthesis using alanine as a novel fuel. Detailed X-ray powder diffraction analyses show 47–96% phase purity in the as-synthesized powder and upon calcination at ～825 °C single-phase material is obtained wherein the nanocrystallinity (crystallite size ～19–24 nm) is retained. Densification studies of the materials are carried out within 900–1100 °C. The coefficient of thermal expansion (CTE) of these cathodes is measured. Electrical conductivity of the cathodes sintered at different temperatures are measured in the temperature range 700–900 °C and correlated with the density of the sintered materials. The electrochemical performances of Ni-YSZ anode-supported SOFC having YSZ electrolyte (～10 μm) with CGO interlayer (～15 μm) are studied with the developed cathodes in the temperature range 700–800 °C using H₂ as fuel and oxygen as oxidant. Highest current density of ～1.7 A/cm² is achieved during testing at 800 °C measured at 0.7 V with a cathode composition of La_0.5Sr_0.5Co_0.8Fe_0.2O₃. Precipitation of nanocrystalline grains over the core grains in porous microstructure of this cathode might be one of the reasons for such high cell performance.     

Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO₂–xWO₃ glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO₂–0.25WO₃ and 0.7TeO₂–0.3WO₃ glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO₂–0.15WO₃ glass. On the basis of the XRD measurements of the 0.85TeO₂–0.15WO₃ glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO₂ (paratellurite), γ-TeO₂ and WO₃ phases were detected in the sample heated to 510 °C and the α-TeO₂ and WO₃ phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO₂–0.15WO₃ glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO₂–0.15WO₃ glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C, respectively.