Luminescent properties of Eu-doped YTaO4 powders

YTaO₄:Eu³⁺ powders were synthesized by a flux method with LiCl. The use of a flux in the synthesis of M′–YTaO₄ facilitated the reaction of raw materials, leading to lowering the heating temperature, but not effective at the high firing temperature. The red emission peaks were observed around 613 nm with an excitation wavelength of 254 nm. Emission peaks were composed of two sets around 613 nm and 590 nm, which originated from ⁵D₀–⁷F₂ and ⁵D₀–⁷F₁, respectively. PL intensity of YTaO₄:Eu³⁺ prepared with a small amount of LiCl (10 wt%) significantly depended on the firing temperatures, while that with a larger amount of LiCl (40 wt%) slightly relied on firing temperatures. The highest PL intensity could be obtained at the firing conditions of 1300 °C and 10 wt% LiCl.     

Effects of crystal structures on luminescent properties of Eu doped Ca–Al–O systems

To investigate the correlations between the structural transformations of calcium aluminates and luminescent properties, Eu doped Ca–Al–O system phosphors were synthesized by solid-state reaction process in H₂ atmosphere with CaCO₃, Al₂O₃, Eu₂O₃, and a flux, H₃BO₃ as starting materials. Various phases such as CaAl₂O₄, CaAl₄O₇, Ca₃Al₂O₆, and Ca₁₂Al₁₄O₃₃ were achieved depending on firing temperature, flux amounts, and the mixing ratio of CaO to Al₂O₃. Among various phases, only CaAl₂O₄ contributed to a strong blue emission at 440 nm with an excitation wavelength of 330 nm.     

Low-temperature sintering of 0.85CaWO4–0.15LaNbO4 ceramics

Microwave dielectric properties of 0.85CaWO₄–0.15LaNbO₄ (CWLN) ceramics were investigated as a function of H₃BO₃, Li₂CO₃ content and sintering temperature. With the co-addition of 3.0 wt.% H₃BO₃–1.0 wt.% Li₂CO₃, the sintering temperature could be effectively reduced from 1150 °C for pure CWLN ceramics to 900 °C without any degradation of dielectric properties. These results are due to the enhancement of the sinterability of CWLN by liquid phase sintering. For the specimens with H₃BO₃–Li₂CO₃ sintered at 900 °C for 3 h, the dielectric constant (K) did not changed remarkably. However, the quality factor (Qf) and the temperature coefficient of resonant frequency (TCF) increased up to y = 1.0 of 3.0 wt.% H₃BO₃–y wt.% Li₂CO₃, and then decreased due to the formation of the secondary phases. Typically, K of 11.8, Qf of 45,200 GHz and TCF of −23.1 ppm/°C were obtained for the specimens of CWLN with 3.0 wt.% H₃BO₃–1.0 wt.% Li₂CO₃ sintered at 900 °C for 3 h.     

Synthesis, sintering and characterization of PNZST ceramics from high-energy ball milling process

Pb_0.99Nb_0.02[(Zr_0.60Sn_0.40)_0.94Ti_0.06]_0.98O₃ powders were synthesized from oxide mixtures of Pb₃O₄, Nb₂O_5, ZrO₂, SnO_2, and TiO₂ using a Fritsch P4™ vario-planetary ball milling system. The perovskite structure of PNZST powder can be obtained well after 14 h milling and crystallite size of the powders greatly reduced to 20–30 nm. The resultant powders have better sinterability characteristics to obtain dense ceramic bodies at low temperature. The dielectric constant of samples shows a maximum at 1150 °C and the PE_MC to PE_SC transformation temperature shifts to higher temperature with an increase in grain size. The field-induced strain reaches maximum longitudinal strain of 0.32% and the P–E hysteresis loop shows much higher squareness at 1150 and 1200 °C. The measured dielectric and antiferroelectric properties of the PNZST ceramics were found to be comparable to those from other processing techniques.     

Synthesis, Characterization, and Luminescent Properties of CaS:Eu Phosphor

Europium-doped calcium sulfide phosphor was synthesized by a co-precipitation method using calcium carbonate and ammonium sulfate as the sources for calcium and sulfide, respectively. Reduction of the precipitated CaSO₄ into CaS was carried out at 550°, 750°, 950°, and 1150°C at a flow rate of H₂ 5 mL/min and N₂ 95 mL/min. The formation of crystalline CaS phase was evident by the X-ray diffraction analysis of the fired products. The phosphor yielded red emission of wavelength 634 nm for the excitation wavelength of 465 nm. The emission intensity of the phosphors increased with increasing firing temperature from 550° to 950°C, but firing at 1150°C showed a dramatic decline in emission due to thermal quenching. The study of the effect of Eu loading level showed a decrease in emission with an increase in loading, and the optimum loading for maximum emission was found to be 0.5 mol%. And it can be possible to generate white light of light emitting devices by mixing with SrGa₂S₄:Eu green phosphor and this phosphor on the 460 nm InGaN blue chip.     

Sintering behavior of Al2TiO5 base ceramics and their thermal properties

Sintering behavior of Al₂TiO₅ without and with various additives and the thermal properties of the sintered material—thermal expansion and decomposition—were investigated. The precursors of Al₂TiO₅ powders were prepared by homogeneous precipitation and coprecipitation. Sintering of pure Al₂TiO₅ gave a fine grained-structure at 1300°C, but resulted in large-grained and cracked microstructures at 1400 and 1500°C. Addition of ZrO₂ or BaO gave fine-grained microstructures with a small increase in thermal expansion. Addition of ZrO₂, BaO or ZrSiO₄, especially ZrSiO₄, was effective in suppressing the thermal decomposition of Al₂TiO₅ at 1100°C. ©     

Dielectric behavior of Nb2O5-doped TiO2/epoxy thick films

TiO₂/epoxy composite thick films containing the TiO₂ powders doped with 4 and 10 vol% Nb₂O₅ heat treated under vacuum at 1050 and 1150 °C, were prepared by the screen printing and curing steps. The Nb₂O₅-doped TiO₂ ceramic bulks demonstrated a higher effective dielectric constant at different densification environments, as compared with pure TiO₂. The dielectric properties of the TiO₂/epoxy thick films were improved if the heat-treated 4 vol% Nb₂O₅-doped TiO₂ powder was incorporated instead of the un-doped and heat-treated 10 vol% Nb₂O₅-doped TiO₂ powders. The disadvantage of the doped TiO₂ having higher dielectric loss tangent could be minimized after its powder was properly treated and mixed with epoxy to form the TiO₂/epoxy composite. A best result with the dielectric constant of 23 and the loss tangent of 0.046 was obtained for the 40 vol% TiO₂/epoxy composite thick films, where the TiO₂ powder was doped with 4 vol% Nb₂O₅ followed by calcination at 1000 °C in air and heat treatment at 1150 °C under vacuum.     

Reaction and characterization studies of an industrial Cr-free iron-based catalyst for high-temperature water gas shift reaction

Activity and stability of an industrial Cr-free iron-based catalyst (NBC-1) for high-temperature water gas shift (WGS) reaction were studied in a fixed-bed reactor under 350 °C, 1 atm, H₂O:gas = 1:1 and 3000 h⁻¹ (dry-gas basis). Physical properties of the NBC-1 catalyst before and after the WGS reaction, the desorption behavior of H₂O, CO, CO₂ and H₂, and surface reaction over the catalyst were characterized by BET, X-ray diffraction (XRD), Mössbauer emission spectroscopy (MES), temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR). The NBC-1 catalyst is active and has excellent thermo-stability even after pretreatment at a high temperature of 530 °C. Its activity and thermo-stability are comparable to those of an UCI commercial Fe-Cr catalyst, C12-4. XRD and MES studies show that iron in the fresh NBC-1 catalyst is present as γ-Fe₂O₃, most of which is converted to Fe₃O₄ during reduction and reaction. Results of TPD demonstrate that adsorbed CO₂ and CO cannot exist on the NBC-1 surface beyond the temperature of 300 °C while higher temperatures (>400 °C) are required to completely desorb H₂O. A redox mechanism of WGS on the NBC-1 surface is proposed based on the TPD and TPSR observations.     

Influence of isovalent and aliovalent substitutions at Ti site on bismuth sodium titanate-based compositions on piezoelectric properties

The effect of isovalent and aliovalent substitutions in Bi_0.5Na_0.485La_0.005TiO₃ (BNLT) compounds were studied within the additive ranges of 0–2.5 at%. The Zr⁴⁺, Nb⁵⁺ and Fe³⁺ ions were selected as the substituents. The modified BNLT compounds were prepared by conventionally mixed-oxide method. The calcination and sintering were performed at the temperatures of 750–850 °C and 1050–1150 °C, respectively. An increase in the substituents contents affected the physical and piezoelectric properties. The BNLT compositions with the addition of 1 at% Zr⁴⁺, Nb⁵⁺ and Fe³⁺ ions exhibited high relative permittivities (_r) at 730, 735 and 660, respectively. The modified-BNLT with an addition of 1.0 at% Fe provided a piezoelectric coefficient (d₃₃) of 155 pC/N, Curie temperature (T_c) of 320 °C and electromechanical coupling factors in planar (k_p) and thickness (k_t) modes of 15 and 45%, respectively.     

Experimental and microkinetic modeling of steady-state NO reduction by H2 on Pt/BaO/Al2O3 monolith catalysts

Experimental results describing the product distribution during the reduction of NO by H₂ on Pt/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts are presented in the temperature range 30–500 °C and H₂/NO feed ratio range of 0.9–2.5. A microkinetic model that describes the kinetics of NO reduction by H₂ on Pt/Al₂O₃ is proposed and most of the kinetic parameters are estimated from either literature data or from thermodynamic constraints. The microkinetic model is combined with the short monolith flow model to simulate the conversions and selectivities corresponding to the experimental conditions. The predicted trends are in excellent qualitative and reasonable quantitative agreement with the experimental results. Both the model and the experiments show that N₂O formation is favored at low temperatures and low H₂/NO feed ratios, N₂ selectivity increases monotonically with temperature for H₂/NO feed ratios of 1.2 or less but goes through a maximum at intermediate temperatures (around 100 °C) for H₂/NO feed ratios 1.5 or higher. Ammonia formation is favored for H₂/NO feed ratios of 1.5 or higher and intermediate temperatures (100–350 °C) buts starts to decompose at a temperature of 400 °C or higher. The microkinetic model is used to determine the surface coverages and explain the trends in the experimentally observed selectivities.     

Regeneration of NOx trap catalysts

We have investigated the regeneration of a nitrated or sulphated model Pt/Ba-based NO_x trap catalyst using different reductants. H₂ was found to be more effective at regenerating the NO_x storage activity especially at lower temperature, but more importantly over the entire temperature window after catalyst ageing. When the model NO_x storage catalyst is sulphated in SO₂ under lean conditions at 650 °C almost complete deactivation can be seen. Complete regeneration was not achieved, even under rich conditions at 800 °C in 10% H₂/He. Barium sulphate formed after the high temperature ageing was partly converted to barium sulphide on reduction. However, if the H₂ reduced sample was exposed to a rich condition in a gas mixture containing CO₂ at 650 °C, the storage activity can be recovered. Under these rich conditions the S²⁻ species becomes less stable than the CO₃²⁻, which is active for storing NO_x. Samples which were lean aged in air containing 60 ppm SO₂ at <600 °C, after regeneration at λ=0.95 at 650 °C, have a similar activity window to a fresh catalyst. It is, therefore, important that CO₂ is present during the rich regenerations of the sulphated model samples (as of course it would be under real conditions), as suppression of carbonate formation can lead to sulphide formation which is inactive for NO_x storage.     

An investigation of the thermal stability and sulphur tolerance of Ag/γ-Al2O3 catalysts for the SCR of NOx with hydrocarbons and hydrogen

The sulphur tolerance and thermal stability of a 2 wt% Ag/γ-Al₂O₃ catalyst was investigated for the H₂-promoted SCR of NO_x with octane and toluene. The aged catalyst was characterised by XRD and EXAFS analysis. It was found that the effect of ageing was a function of the gas mix and temperature of ageing. At high temperatures (800 °C) the catalyst deactivated regardless of the reaction mix. EXAFS analysis showed that this was associated with the Ag particles on the surface of the catalyst becoming more ordered. At 600 and 700 °C, the deactivating effect of ageing was much less pronounced for the catalyst in the H₂-promoted octane-SCR reaction and ageing at 600 °C resulted in an enhancement in activity for the reaction in the absence of H₂. For the toluene + H₂-SCR reaction the catalyst deactivated at each ageing temperature. The effect of addition of low levels of sulphur (1 ppm SO₂) to the feed was very much dependent on the reaction temperature. There was little deactivation of the catalyst at low temperatures (≤235 °C), severe deactivation at intermediate temperatures (305 and 400 °C) and activation of the catalyst at high temperatures (>500 °C). The results can be explained by the activity of the catalyst for the oxidation of SO₂ to SO₃ and the relative stability of silver and aluminium sulphates. The catalyst could be almost fully regenerated by a combination of heating and the presence of hydrogen in the regeneration mix. The catalyst could not be regenerated in the absence of hydrogen.     

Effect of temperature on the porosity, microstructure, and properties of porous La0.8Sr0.2MnO3 cathode materials

Porous La_0.8Sr_0.2MnO₃ cathode materials were prepared by the gelcasting technology. Carbon was employed to produce pores. It is shown that the open porosity decreases with increasing temperature. The proper sintering temperature is 1100 °C and the median pore size of the sample obtained at this temperature is about 460 nm. The microstructure indicates that the grain grows as the sintering temperature increases, which leads to the decrease of the number of open pores. Generally, the pores are located at multi-grain boundaries. Some closed pores appear in the sample prepared at 1100 °C and more. Both the conductivity and the interface bonding between La_0.8Sr_0.2MnO₃ and YSZ get better as the temperature increases. When the sintering temperature is more than 1250 °C, La and Mn ions begin to diffuse into YSZ, and therefore interface reactions happen. According to ln(σT)  1/T curves, E_a was calculated to be 10.18 kJ/mol.     

Low-temperature H2 and N2 transport through thin Pd66Cu34Hx layers

The single gas H₂ and N₂ permeability of a 4 μm thick dense fcc-Pd₆₆Cu₃₄ layer has been studied between room temperature and 510 °C and at pressure differences up to 400 kPa. Above 50 °C the H₂ flux exhibits an Arrhenius-type temperature dependence with J_H2=(5.2±0.3) mol m⁻² s⁻¹ exp[(−21.3 ± 0.2) kJ mol⁻¹/(R·T)]. The hydrogen transport rate is controlled by the bulk diffusion although the pressure dependence of the H₂ flux deviates slightly from Sieverts’ law. A sudden increase of the H₂ flux below 50 °C is attributed to embrittlement.     

Sintering behavior and microwave dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics sintered under air and N2 atmosphere

The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) and Bi₂(Zn_1/3Nb_2/3−xV_x)₂O₇ (BZNV, x = 0.001) sintered under air and N₂ atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V₂O₅ and N₂ atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N₂ atmosphere with _r = 76.1, Q = 850 and Q_f = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with _r = 76.7, Q = 890 and Q_f = 3580 GHz. The temperature coefficient of resonant frequency τ_f was not obviously influenced by the different atmospheres. For BZN ceramics the τ_f was −79.8 ppm/°C while τ_f is −87.5 ppm/°C for BZNV ceramics.     

Structure evolution of nanocrystalline CeO2 and CeLnOx mixed oxides (Ln = Pr, Tb, Lu) in O2 and H2 atmosphere and their catalytic activity in soot combustion

This paper reports results of studies on structure and activity in soot combustion of nanocrystalline CeO₂ and CeLnOx mixed oxides (Ln = Pr, Tb, Lu, Ce/Ln atomic ratios 5/1). Nano-sized (4–5 nm) oxides with narrow size distribution were prepared by a microemulsion method W/O. Microstructure, morphology and reductivity of the oxides annealed up to 950 °C in O₂ and H₂ were analyzed by HRTEM, XRD, FT-IR, Raman spectroscopy and H₂-TPR. Obtained mixed oxides had fluorite structure of CeO₂ and all exhibited improved resistance against crystal growth in O₂, but only CeLuOx behaved better than CeO₂ in hydrogen.

The catalytic activity of CeO₂, CeLnOx and physical mixtures of CeO₂ + Ln₂O₃ in a model soot oxidation by air was studied in “tight contact” mode by using thermogravimetry. Half oxidation temperature T_1/2 for soot oxidation catalysed by nano-sized CeO₂ and CeLnOx was similar and ca. 100 °C lower than non-catalysed oxidation. However, the mixed oxides were much more active during successive catalytic cycles, due to better resistance to sintering. Physical mixtures of nanooxides (CeO₂ + Ln₂O₃) showed exceptionally high initial activity in soot oxidation (decrease in T_1/2 by ca. 200 °C) but degraded strongly in successive oxidation cycles. The high initial activity was due to the synergetic effect of nitrate groups present in highly disordered surface of nanocrystalline Ln₂O₃ and enhanced reductivity of nanocrystalline CeO₂.     

Corrosion of selected ceramic materials in hot gas environment

The temperature dependence of the hot gas corrosion behaviour of various ceramic materials (Al₂O₃, ZrO₂ (Y-TZP), mullite, ZrSiO₄ and YAG) was investigated. The tests were performed in a high temperature burner rig at temperatures between 1200 °C and 1500 °C, a total pressure of 1 atm with a water vapour partial pressure of 0.24 atm, a gas flow velocity of 100 m/s and test times of about 130–300 h.

ZrO₂ (Y-TZP) showed absolutely no corrosion, however, a very high susceptibility to thermal shock and phase transformation was observed.

The other materials suffered degradation above 1300 °C. This was the consequence of the formation and evaporation of volatile hydroxides (e.g. Si(OH)₄ and Al(OH)₃). YAG showed a low corrosion rate and the formation of a protective surface layer. The corrosion susceptibility of these materials was found to be higher with increasing temperature.

Thermochemical calculations of the partial pressure of volatile species formed in reaction with water vapour, affirm the observed differences in corrosion behaviour.     

Integrated high temperature gas cleaning: Tar removal in biomass gasification with a catalytic filter

A nickel-based catalytic filter material for the use in integrated high temperature removal of tars and particles from biomass gasification gas was tested in a broad range of parameters allowing the identification of the operational region of such a filter. Small-scale porous alumina filter discs, loaded with approximately 2.5 wt% Al₂O₃, 1.0 wt% Ni and 0.5 wt% MgO were tested with a particle free synthetic gasification gas with 50 vol% N₂, 12 vol% CO, 10 vol% H₂, 11 vol% CO₂, 12 vol% H₂O, 5 vol% CH₄ and 0–200 ppm H₂S, and the selected model tar compounds: naphthalene and benzene. At a typical face velocity of 2.5 cm/s, in the presence of H₂S and at 900 °C, the conversion of naphthalene is almost complete and a 1000-fold reduction in tar content is obtained. Technically, it would be better to run the filter close to the exit temperature of the gasifier around 800–850 °C. At 850 °C, conversions of 99.0% could be achieved in typical conditions, but as expected, only 77% reduction in tars was achieved at 800 °C.

Conversion data can be reasonably well described with first order kinetics and a dominant adsorption inhibition of the Ni sites by H₂S. The apparent activation energies obtained are similar to those reported by other investigators: 177 kJ/mol for benzene and 92 kJ/mol for naphthalene. The estimated heat of adsorption of H₂S is 71 kJ/mol in the benzene experiments and 182 kJ/mol in the naphthalene experiments, which points at very strong adsorption of H₂S. Good operation of the present material can hence only be guaranteed at temperatures above 830 °C mainly due to the strong deactivation by H₂S at lower temperatures.     

Effects of the sintering atmosphere on Nb-based dielectrics

The effects of a forming atmosphere on the stability, the sintering and the dielectric properties of Ba₅Nb₄O₁₅, BaNb₂O₆, ZnNb₂O₆ and Zn₃Nb₂O₈ ceramics were investigated, because of the primary importance of the sintering atmosphere in relation to copper sintering. These Nb-based materials were sintered in air and in Ar/H₂10%. Zn-containing samples are very sensitive to the reductive atmosphere. ZnO volatilises at 800–850 °C and the resulting compound does not exhibit the expected properties. BaNb₂O₆ and Ba₅Nb₄O₁₅ are more stable in term of relative weight loss. Nevertheless, the phase analysis reveals a modification of the BaNb₂O₆ phase, what induces the degradation of the dielectric property stability versus temperature. The properties of Ba₅Nb₄O₁₅ are not modified by a sintering in reductive atmosphere. A relative permittivity of 38.8, a permittivity temperature coefficient of −150 ppm °C⁻¹ and an insulating resistivity of 10^10.9 Ω cm were obtained for this latter.     

Improvement of response characteristics of TiO2 humidity sensors by simultaneous addition of Li2O and V2O5

Porous TiO₂ ceramics were prepared by adding various amounts of Li₂O and V₂O₅ and the humidity sensitivity of the resulting ceramics was investigated by means of electrical measurements, scanning electron microscopy and mercury intrusion porosimetry. Simultaneous addition of Li₂O and V₂O₅ to TiO₂ enabled sintering at temperatures as low as 700 °C and also decreased the impedance of the ceramics. Furthermore, in the ceramics including these additives simultaneously, excellent humidity sensitivity as well as good response characteristics were observed. The microstructures of these ceramics depended on the firing temperature and the amount and ratio of Li₂O/V₂O₅, and optimum humidity sensitivity was observed for the sample including both 0.25 mol% Li₂O and 0.75 mol% V₂O₅ fired at 700 °C. These results indicated that the humidity sensitivity and its response characteristics were closely related to the microstructure, and that improving the uniformity of microstructure is important for improving humidity sensitivity and its response characteristics.