Electromechanical properties and defect chemistry of high-temperature piezoelectric materials

Langasite and gallium phosphate are shown to exhibit piezoelectrically stimulated bulk acoustic waves up to at least 1,400 and 900 °C, respectively. Most critical issues are stoichiometry changes due to, e.g. low oxygen partial pressures, and high losses. Therefore, the paper discusses the atomistic transport and defect chemistry of those crystals and correlates them with the electromechanical properties. First, the defect chemistry of langasite is investigated. As long as the atmosphere is nearly hydrogen-free, the transport of charge carriers is governed by oxygen movement. A dominant role of hydrogen is observed in hydrogenous atmospheres. Based on the developed defect model, donors are expected to suppress the oxygen vacancy concentration and, thereby, the loss in langasite. The prediction is proven by niobium doping and found to be valid. A one-dimensional physical model of thickness shear mode resonators is summarized. The analysis of the resonance spectra showed that the loss of the resonators can be described satisfactorily by mechanical and electrical contributions expressed as effective viscosity and bulk conductivity, respectively. The mechanical loss in langasite is significantly impacted by the electrical conductivity due to the piezoelectric coupling. The effect of the piezoelectric coupling on the loss is negligible for gallium phosphate since it shows an extremely low electrical conductivity.     

Sensor application-related defect chemistry and electromechanical properties of langasite

The operation of langasite (La₃Ga₅SiO₁₄) resonators as sensors at elevated temperature and controlled atmospheres is examined. This paper focuses on mapping the regimes of gas-insensitive operation of uncoated langasite resonators and the correlation to langasite's defect chemistry for temperatures up to 1000 °C. As a measure of sensitivity, the fundamental resonant mode at 5 MHz is estimated to be determined to within ± 4 Hz by network analysis for resonators operated in air at temperatures below 1000 °C. The calculated frequency shift induced by redox-related reactions in langasite only exceeds the limit of ± 4 Hz below p_O2 ≈ 10^− 17 bar at 1000 °C, below 10^− 24 bar at 800 °C and below 10^− 36 bar at 600 °C. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen-containing atmospheres applied here, langasite can be regarded as a stable resonator material above oxygen partial pressures of about 10^− 13 and 10^− 20 bar at 800 and 600 °C, respectively.     

Rearrangement of the Ultrasmooth Surface of La3Ga5SiO14 Crystals at Heating

The morphology and phase composition of the surface of La₃Ga₅SiO₁₄ (langasite) crystals at annealing in a temperature range 1000–1200°C have been studied using electron and atomic force microscopy. It has been shown that trigonal lanthanum oxide (La₂O₃) crystals with sizes to 3–4 μm, as well as a microstructure with sizes to 50 μm with gallium excess, with the approximate composition of 15 mol % La₂O₃, 65 mol % Ga₂O₃, and 20 mol % SiO₂ are formed on the surface of langasite crystals annealed in air at temperatures above 1100°C. Possible reasons for thermal destruction of the compound can be a significant rearrangement of the disordered crystal structure of langasite caused by the interaction with air oxygen and under the intense surface diffusion of atoms of the crystal, as well as the incongruent character of melting of the La₃Ga₅SiO₁₄ compound. The revealed thermal destruction of the surface of langasite crystals should be taken into account when using this material to fabricate piezoelectric elements for operation at high temperatures.

     

Crystal microstructure of annealed nanocrystalline Chromium studied by synchrotron radiation diffraction

The microstructure of electrodeposited nanocrystalline chromium (n-Cr) was studied by using synchrotron radiation (SR) diffraction, SEM, TEM, and EDX techniques. The as-prepared n-Cr samples show the standard bcc crystal structure of Cr with volume-averaged column lengths varying from 25 to 30 nm. The grain growth kinetics and the oxidation kinetics were studied by time resolved SR diffraction measurements with n-Cr samples annealed at 400, 600, and 800 °C. The grain growth process is relatively fast and it occurs within the first 10 min of annealing. The final crystallite size depends only on the annealing temperature and not on the initial grain size or on the oxygen content. The final volume-averaged column lengths observed after 50 min annealing are 40(4), 80(1), and 120(2) nm for temperatures 400, 600, and 800 °C, respectively. It is shown that annealing ex situ of n-Cr at 800 °C both under vacuum and in air gives a grain growth process with the same final crystallite sizes. The formation of the Cr₂O₃ and CrH phases is observed during annealing.     

Optimization of calcination temperature of lead-free BiFeO3–BaTiO3 high-temperature piezoceramics

0.7BiFeO₃-0.3BaTiO₃+1mol% MnO₂(0.7BFO-0.3BTO) ceramics were synthesized by conventional solid-state powder method under different calcination temperatures (T_cal) between 770 and 830 °C. The phase structure, microstructure, and ferroelectric and piezoelectric properties changed greatly depending on the applied T_cal. Benefitting from the formation of low defect levels and large grain size and an appropriate morphotropic phase boundary (MPB) with the rhombohedral-to-pseudocubic phase ratio = 49.1 : 50.9, BFO-BTO ceramics calcined at 785 °C showed the best ferroelectric, piezoelectric, and insulating properties (P_r = 23.1 μC/cm², E_C = 25.8 kV/cm, d₃₃ = 167.8 pC/N, k_p = 0.342%). Above T_cal = 800 °C, however, the ferroelectric and piezoelectric properties deteriorated because volatilization of Bi and reduction of Fe caused a poor insulating property and high degree of chemical inhomogeneity. Moreover, the ceramics calcined at 785 °C showed a high Curie temperature (T_C) of 509.2 °C and excellent thermal aging resistance of d₃₃ up to 450 °C, demonstrating great potential for use in high-temperature applications.     

Three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films

A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d ₃₃ coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140°C keep their piezoelectric activity up to at least 120°C and retain 70% of their initial d ₃₃ even at 130°C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz.     

Impedance spectroscopy study of gadolinia-doped ceria

The electrical properties of Gd_0.1Ce_0.9O_1.95 were investigated in various gas atmospheres using a.c. impedance spectroscopy. In dry oxygen, impedance spectra consisted of two arcs at low temperatures corresponding to bulk and grain boundary behaviour, respectively, and only one arc at high temperatures (T>550 °C). The results showed a high oxygen ion conductivity in oxygen atmosphere more than 10⁻² S/cm at 600 °C. In water atmospheres, the bulk arc in dry oxygen was reduced by 30 %, and an additional arc corresponding to the grain boundary effect appeared. It seems that the presence of water may decrease the bulk resistance, but causes an additional grain boundary resistance and thus increase in the total resistance. In hydrogen atmosphere, a conductivity enhancement up to one order of magnitude was observed. In hydrogen there is a reduction of Ce⁴⁺ to Ce³⁺ in the sample, which involves defect formation generating e.g. electrons, additional oxygen vacancies and at the same time protons in the lattice. These new charge carriers are responsible for a significant conductivity enhancement. Therefore, in a hydrogen atmosphere the material is a mixed (oxygen ion + proton + electron) conductor. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion was carried out in this study to investigate the influences of gas flow rate, oxidative atmosphere, and inert porous medium involvement on the properties of products. The experiments were carried out at 300–600 °C under atmospheres of N₂, 5% O₂/95% N₂, 10% O₂/90% N₂, and 15% O₂/85% N₂, with Darcy flow rates of 1.0 and 3.5 cm/s, respectively, with dried sewage sludge loaded individually or as a mixture with sand. As a result, both the increment of gas flow rate and involvement of sand leaded to lower yields of char and higher yields of bio-oil and gas under N₂ at temperature of ≤500 °C, due to the enhanced efficiency of pyrolysis reaction and gas transportation. However, when temperature increased to 600 °C, the influencing trends on product distributions changed due to the mechanisms of secondary cracking reaction and volatile-char interaction. The involvement of oxygen in fraction of ≤15 vol% at temperatures of 400–500 °C would lead to the intense decreasing yields of char and bio-oil, and increasing yield of the gaseous (dominated by CO₂ and CO), due to the involved oxidation reaction during pyrolysis. Both increment of temperature and oxygen fraction would lead to the delay of ignition and the increase of activation energy of the produced char, except for that of char produced at 400 °C under 5% O₂/95% N₂, whose calculated activation energy was lower and volatile content was higher compared to that of char produced from pyrolysis at 400 °C under N₂. The bio-oil from pyrolysis under N₂ was dominated by aliphatic acids, phenols, steroids, amides, and indoles, etc., and the involvement of partial oxidation would lead to the weakened formation of aromatics, phenols, and S/Cl/F-containing compounds in bio-oil.     

Large blue shift in the optical band-gap of sol-gel derived Ba0.5Sr0.5TiO3 thin films

In this paper, we report and analyze the large blue shift in the optical band-gap of sol-gel derived Ba_0.5Sr_0.5TiO₃ (BST) thin films. BST films of different thickness (150 nm, 320 nm and 480 nm respectively) were deposited layer by layer onto fused quartz substrates by a spin coating technique. The drying temperature for individual layers (pre-sintering temperature) was varied as 400, 500 and 600 ^°C. A large blue shift in the band-gap was observed (with a value 4.70 eV compared to the bulk value of 3.60 eV) for films pre-sintered at 400 ^°C, which decreased with increase in the pre-sintering temperature. To date such blue shifts have been attributed to grain size reduction, stress and the amorphous nature of the films. Here, the blue shift has been correlated with the presence of charge carriers generated by oxygen vacancies and explained on the basis of the Burstein-Moss effect.     

Electrical behaviour of LSGM–LSM composite cathode materials

Composite cathode materials were prepared by mixing La_0.83Sr_0.17Ga_0.83Mg_0.17O_2.83 (LSGM) and La_0.8Sr_0.2MnO₃ (LSM) powders fired at 1300 °C. Several compositions were set up containing 1, 5, 25, 50, 75 wt.% of LSM. Their microstructure and electrical behaviour were investigated by XRPD, SEM/EDS and EIS. In composites containing 50 and 75 wt.% of LSM, the electronic contribution to conductivity is predominant, then there is only a single point at the low frequency end of the Nyquist plot. On the contrary, in the composites with up to 25 wt.% of LSM, there is a significant amount of ionic transport, then the IS spectra show complex features: at least three different arcs can be devised and their interpretation depends upon temperature. LSGM bulk and grain boundary conductivity, as well as interface polarization between the ionic (LSGM) and electronic (LSM) phases can be separated at temperatures below 600 °C; total LSGM contribution, i.e. bulk plus grain boundary, LSGM–LSM interface and electrode polarizations are attributed above 600 °C.     

Optical constants of vacuum annealed radio frequency (RF) magnetron sputtered zinc oxide thin films

Zinc oxide thin films were deposited by radio frequency magnetron sputtering. The films were annealed in vacuum at temperatures of 400, 600, and 800 °C. The influence of annealing on the structural, chemical, and optical properties of the films was investigated. From a structural point of view, the films were highly oriented, with columnar microstructure. Chemical analysis indicated that the films were sub-stoichiometric, and that the concentration of oxygen vacancies was enhanced upon annealing. The films were highly transparent in the visible and near-infrared spectral regions. Transparency was reduced as the annealing temperature was increased. The refractive index and extinction coefficient, in the transparent regions of the films, were derived from transmittance measurements. The refractive index manifested variation that was affected by crystallite size, roughness and defect concentration. The extinction coefficient of the as-deposited films and those annealed at 400 °C was negligible. However, the films annealed at 600 or 800 °C had much larger values of the extinction coefficient due to increased absorption or scattering. The absorption coefficient and optical band gap of the films were derived from spectrophotometric measurements. The absorption coefficient showed progressive increase with the annealing temperature. However, the band gap did not show significant variation.     

High c-axis preferred orientation ZnO thin films prepared by oxidation of metallic zinc

Thin films of zinc oxide were grown on glass substrates by thermal oxidation. The metallic zinc films were thermally oxidized at different temperatures ranging from 300 to 600 °C to yield ZnO thin films. The structural property of the thin films was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray diffraction measurements showed that the films oxidized at 300 °C were not oxidized entirely, and the films deposited at 600 °C had better crystalline quality than the rest. When the oxidation temperature increased above 400 °C, the films exhibited preferred orientation along (002) and high transmittance ranging from 85% to 98% in vis–near-infrared band. Meanwhile, the films showed a UV emission at about 377 nm and green emission. With the increasing of oxidation temperature, the intensity of green emission peak was enhanced, and then decreased, disappearing at 600 °C, and the case of UV emission increased. Furthermore, a strong green emission was observed in the film sintered in pure oxygen atmosphere.     

Phase transformation and oxygen permeation properties of partially substituted strontium cobaltite — Influences of B-site partial substitution —

A group of B-site partially substituted strontium cobaltites Sr_0.9Ca_0.1Co_0.9B'_0.1O_2.5+δ (B'=Mn, Co, Ni, In, Fe and Al) were examined for phase transformation and oxygen permeation properties. As revealed by XRD analysis, these oxides had 2H-BaNiO₃-type structure (B'=Mn, Co, Ni and In) or brownmillerite-type structure (B'=Fe and Al) and transformed into perovskite-type structure at elevated temperatures (750 °C or above). The transformation temperature was dependent on the B-site substituent and agreed well with the switching-on temperature of oxygen permeation for the same oxide. Due to a lowering in transformation temperature, the Fe- or Al-doped oxide was made oxygen permeable in the lower temperature range 750–850 °C where the undoped oxide remained non-permeable. It was also found that for a series of Fe-partially substituted samples Sr_0.9Ca_0.1Co_1−xFe_xO_2.5+δ the transformation temperature lowered with increasing x up to 0.1, while, for x≥0.2, perovskite-type structure tended to be stabilized at room temperature. TGDTA analysis revealed that phase transformation was accompanied by absorption or desorption of oxygen. Based on the correlation between the oxygen permeability and the membrane thickness, the rate-determining step of oxygen permeation in the present system was assumed to be of the bulk diffusion of oxide ion vacancies.     

Investigations on the sulphur incorporation in sputtered Cu,In multilayer precursors

High quality CuInS₂ layers have been grown using a two step process. Multiple Cu, In bilayers were deposited by RF magnetron sputtering on Mo-coated glass substrates followed by sulphurisation using elemental sulphur in a closed graphite box at different temperatures up to 550°C. The layers were characterised to study their structure and composition using XRD, SEM and EDAX. More sulphur was incorporated at lower annealing temperatures with sulphur content higher than 50 at.%, while the Cu/In ratio ranges between 0.9 to 1.1. As the annealing temperature increases beyond 400°C, the composition of sulphur decreases to 46 at.% with an incorporation of Mo in the layers. The secondary phases observed at different annealing temperatures were identified and reported. Polycrystalline, single phase and densely packed CuInS₂ layers with (1 1 2) preferred orientation were obtained for annealing temperatures of about 350°C.     

Soft chemistry routes for synthesis of rare earth oxide nanoparticles with well defined morphological and structural characteristics

Phosphors of (Y_0.75Gd_0.25)₂O₃:Eu³⁺ (5 at.%) have been prepared through soft chemistry routes. Conversion of the starting nitrates mixture into oxide is performed through two approaches: (a) hydrothermal treatment (HT) at 200 °C/3 h of an ammonium hydrogen carbonate precipitated mixture and (b) by thermally decomposition of pure nitrate precursor solution at 900 °C in dispersed phase (aerosol) within a tubular flow reactor by spray pyrolysis process (SP). The powders are additionally thermally treated at different temperatures: 600, 1000, and 1100 °C for either 3 or 12 h. HT—derived particles present exclusively one-dimensional morphology (nanorods) up to the temperatures of 600 °C, while the leaf-like particles start to grow afterward. SP—derived particles maintain their spherical shape up to the temperatures of 1100 °C. These submicron sized spheres were actually composed of randomly aggregated nanoparticles. All powders exhibits cubic Ia-3 structure (Y_0.75Gd_0.25)₂O₃:Eu and have improved optical characteristics due to their nanocrystalline nature. The detailed study of the influence of structural and morphological powder characteristics on their emission properties is performed based on the results of X-ray powder diffractometry, scanning electron microscopy, X-ray energy dispersive spectroscopy, transmission electron microscopy, and photoluminescence measurements.     

Water vapour solubility and conductivity study of the proton conductor BaCe(0.9 − x)ZrxY0.1O(3 − δ)

The perovskite BaCe_(0.9 ? x)Zr_xY_0.1O_(3 ? δ) has been prepared by solid state reaction at 1400 °C and conventional sintering at 1700 °C. Water uptake experiments performed between 400 and 600 °C, at a water vapour pressure of 0.02 atm, provide data on the concentration of protons incorporated in the sample. The direct current conductivity has been measured as a function of oxygen partial pressure, at a water vapour partial pressure of 0.015 atm. The total conductivity has been resolved into a p-type and an ionic component using a fitting procedure appropriate to the assumed defect model. An estimation of the protonic component was made by assuming a conductivity isotope effect between 1.4 and 1.8. The total conductivity, obtained using impedance spectroscopy has been measured as a function of temperature in the water and heavy water exchanged states. The activation energy has been found to be 0.56 eV to 0.59 eV in the water exchanged state with values 0.03 to 0.04 eV higher in the heavy water exchanged state. Impedance spectra measured at 200 °C showed a reduction in grain boundary resistivity with increasing cerium content. The stability of the compounds to carbon dioxide has been studied by thermogravimetry.     

Effects of annealing on the luminescent properties from defects formed in electron-irradiated GaN

The effects of annealing on the luminescence properties of electron-irradiated GaN were investigated by photoluminescence (PL), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Our results indicate that the yellow luminescence (YL) remains disappear after all annealing treatments and shows a non-linear dependence on the annealing temperature up to 800 °C. The annealing process can be divided into two main stages associated with irradiation defects. At annealing temperatures up to 600 °C, the behavior of YL can be attributed to the net effects of V_GaO_N complex formation caused by the migration of gallium vacancies (V_Ga) and V_GaO_N complex dissociation due to the disappearance of gallium vacancies. At annealing temperatures approaching 800 °C, the incorporation of carbon into the group-V sublattice becomes the dominant factor in the appearance of YL. In addition, experimental results of PL and SEM show that a small proportion of irradiation damage still exists in GaN after annealing at 800 °C.     

Structural and electrochemical studies on thin film LiNi0.8Co0.2O2 by PLD for micro battery

Thin film of LiNi_0.8Co_0.2O₂ (LNCO) has been prepared by Pulsed Laser Deposition (PLD) technique at various post annealing temperatures. XRD results of LNCO thin film deposited on both Pt and Si substrates reveal relatively good crystalline nature at 500 °C which is in good agreement with the electrochemical results. ICP-AES composition analysis indicates 10 to 5% Li loss in the post annealed (400–700 °C) LNCO/Pt thin films; however the as prepared LNCO/Pt films show 17% excess of Li which are comparable with the LNCO target results. SEM analysis indicates phase separation at 600 °C and porous nature at 700–800 °C for LNCO/Pt films. Cyclic voltammetry (CV) scans of LiNi_0.8Co_0.2O₂ film post annealed at 500 °C show a pair of main cathodic and anodic peaks at 3.64 and 3.4 V, respectively with a narrow peak separation reveals good stability upon cycling. Whereas the LNCO films annealed at 600 °C and 700 °C indicate an additional anodic peak at lower potential besides a pair of major peaks which may be due to the phase separated morphology as evidenced from SEM analysis. Based on the structural and electrochemical results, a lithium-ion micro cell has been constructed with LNCO/Li_3.4V_0.6Si_0.4O₄(LVSO)/SnO configuration with the thickness of 1.535 µm and its electrochemical properties have been studied.     

High-temperature stability of the phase composition of langasite family crystals

Thermal stability of the phase composition of langasite family crystals is studied by means of high-temperature X-ray diffractometry upon heating in vacuo and in air. Upon heating above 1000°C in vacuo, the partial decomposition of the initial phase is observed in La₃Ga₅SiO₁₄, La₃Ta_0.5Ga_5.5O₁₄, and Ca₃TaGa₃Si₂O₁₄ crystals; this is accompanied by the emergence of gallium-depleted oxides of the main elements, due to the formation of volatile gallium protoxide and the loss of gallium in the surface layer of the crystals. Langasite family crystals are found to be stable when annealed in air up to a temperature of 1200°C.