Synthesis, densification and electrical properties of strontium cerate ceramics

Powders of pure and 5% ytterbium substituted strontium cerate (SrCeO₃/SrCe_0.95Yb_0.05O_3−δ) were prepared by spray pyrolysis of nitrate salt solutions. The powders were single phase after calcination in nitrogen atmosphere at 1100 °C (SrCeO₃) and 1200 °C (SrCe_0.95Yb_0.05O_3−δ). Dense SrCeO₃ and SrCe_0.95Yb_0.05O_3−δ materials were obtained by sintering at 1350–1400 °C in air. Heat treatment at 850 and 1000 °C, respectively, was necessary prior to sintering to obtain high density. The dense materials had homogenous microstructures with grain size in the range 6–10 μm for SrCeO₃ and 1–2 μm for SrCe_0.95Yb_0.05O_3−δ. The electrical conductivity of SrCe_0.95Yb_0.05O_3−δ was in good agreement with reported data, showing mixed ionic–electronic conduction. The ionic contribution was dominated by protons below 1000 °C and the proton conductivity reached a maximum of 0.005 S/cm above 900 °C. In oxidizing atmosphere the p-type electronic conduction was dominating above 700 °C, while the contribution from n-type electronic conduction only was significant above 1000 °C in reducing atmosphere.     

Microstructure development and properties of novel Ba-doped phase Sialon ceramics

Herein, we report the microstructure and properties of the newly developed near monophasic S-Sialon ceramic, based on the composition of Ba₂Si_12−xAl_xO_2+xN_16−x (x = 20.2). Appropriate amount of the precursor powders (BaCO₃, -Si₃N₄, AlN, Al₂O₃) with a targeted composition of BaAlSi₅O₂N₇ was ball milled and hot pressed to full density in the temperature range of 1600–1750 °C for 2 h in nitrogen atmosphere. Extensive transmission electron microscopy (TEM) study has been conducted to understand the microstructure development and characterise the various morphological features in hot pressed S-Sialon. The sintering mechanism is based on the liquid phase sintering route, which involves the formation of a Ba–Al silicate liquid (<5%) with dissolved nitrogen at intergranular pockets. The experimental observation suggests that the S-phase crystallises in elongated platelet morphology with preferred growth parallel to the orthorhombic ‘c’ axis and primary facet planes parallel to (1 0 0) and (0 1 0). The Ba-S-phase ceramic has an acoustically measured Young modulus of 210–230 GPa, a hardness of 13 GPa and a fracture toughness of 4 MPa m^1/2, little lower than typical of a ceramic with morphologically anisotropic grains contributing to bridging and pullout mechanisms.     

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.     

Synthesis of nanophase tungsten carbide by electrical discharge machining

Tungsten carbide nanopowders were synthesized successfully by electric discharge machining followed by annealing under a nitrogen atmosphere. The tungsten workpieces were initially melted and evaporated on the working surface during the electric discharge machining process, and then the tungsten powders were reacted with the carbon electrode and the working medium of kerosene to form the nanocrystalline WC_1−x powders. The powders produced were characterized by XRD, SEM, and TEM. When annealing the powders under an N₂ atmosphere, the cubic phases of WC_1−x gradually changed to hexagonal W₂C and then were transformed fully to nanocrystalline hexagonal WC at 1200 °C, with the nanocrystalline tungsten carbide encapsulated in a carbon shell. On the other hand, under an H₂ atmosphere, the WC_1−x phase changed via a W₂C phase to reduced powders of pure tungsten at 1000 °C or were reduced directly from WC_1−x to elemental W.     

Synthesis and characterization of Li ion conducting La2/3−xLi3xTiO3 by a polymerizable complex method

Ultrafine lithium ion conducting La_2/3−xLi_3xTiO₃ (x = 0.11, LLT) powder was synthesized by a simple polymerizable complex method based on the Pechini-type process. The formation mechanism, homogeneity and microstructure of the samples were investigated by thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD analysis indicated the formation of pure perovskite-type phase. The powder synthesized at a temperature as low as 900 °C in a much shorter time than solid-state reaction method was well crystallized. The lithium ion conductivity of the LLT ceramics sintered at 1200 °C was found to be 9 × 10⁻⁴ S/cm at room temperature.     

Processing of hot pressed Al2O3–Cr2O3/Cr-carbide nanocomposite prepared by MOCVD in fluidized bed

Nanosized particles dispersed uniformly on Al₂O₃ particles were prepared from the decomposition of precursor Cr(CO)₆ by metal organic chemical vapor deposition (MOCVD) in a fluidized chamber. These nanosized particles consisted of Cr₂O₃, CrC_1−x, and C. A solid solution of Al₂O₃–Cr₂O₃ and an Al₂O₃–Cr₂O₃/Cr₃C₂ nanocomposite were formed when these fluidized powders were pre-sintered at 1000 and 1150 °C before hot-pressing at 1400 °C, respectively. In addition, an Al₂O₃–Cr₂O₃/Cr-carbide (Cr₃C₂ and Cr₇C₃) nanocomposite was formed when the particles were directly hot pressed at 1400 °C. The interface between Cr₃C₂ and Al₂O₃ is non-coherent, while the interface between Cr₇C₃ and Al₂O₃ is semi-coherent.     

On the surface morphology of solution annealed Co1−xO–MgO—Effects of directional dislocation exposure and Co1−xO condensation

The sintered Co_1−xO ceramics with or without 20 mol% MgO solid solution in the rock salt type structure were annealed in the temperature range of 400–1500 °C in air for surface morphology development study. Electron microscopic observations indicated the MgO component considerably suppressed the thermal etching and the nucleation of the Co-rich spinel as expected. Surprisingly, prolonged annealing at 1500 °C caused anisotropic development of the {1 1 1}/{1 0 0}-faceted etch pits/hillocks from the cubic crystal system, which can be rationalized by the predominant exposure of 1 1 0 oriented dislocations on the {1 1 1} surfaces. Meanwhile, sublimation–condensation at this temperature caused cube-like Co_1−xO crystallites to deposit preferentially on the (1 0 0) surface following parallel or 45° off crystallographic relationship via Brownian motion of the crystallites.     

In-Situ Carbon Content Adjustment in Polysilazane Derived Amorphous SiCN Bulk Ceramics

The present paper is concerned with the in-situ carbon content adjustment in amorphous bulk silicon carbonitride (SiCN) ceramic matrices prepared by the polymer to ceramic transformation of cross-linked and compacted poly(hydridomethyl)silazane powders. Heat treatment in inert (Ar) or reactive atmosphere (ammonia, or mixed Ar/NH₃ with different volume ratio of ammonia) was used for carbon content adjustment. Isothermal annealing steps in Ar and/or mixed atmospheres at various intermediate temperatures were also included into the pyrolysis schedule (i) to adjust the final carbon content, (ii) to control outgassing of low molecular reaction products like methane or hydrogen from the matrix during polysilazane decomposition and thus (iii) to avoid cracking of the pressed polymer powders. Optimal annealing temperature for carbon content adjustment was found to be in the range between 500 and 550°C. Increasing NH₃ contents from 10 to 50 vol% in the pyrolysis atmosphere as well as enhanced transient annealing temperature and time promote carbon reduction. In contrast intermediate isothermal annealing in Ar at 500 up to 600°C results in pronounced formation of Si–C bonds and in increased carbon contents after the final pyrolysis process. Depending on the pyrolysis conditions, flawless bulk specimens with carbon contents ranging from 0·3 up to 16·2 wt% were obtained. Different possible chemical reactions are considered to explain the generation of the particular Si(C)N compositions found. ©     

Uber die quantitative bestimmung der kristallgitterstorungen von bleipulver in zusammenhanng mit dessen adsorptionseigenschaften

The lattice distortion ±(δ d/d) of lead powder has been studied by the Hall method. This distortion has a decisive influence on the adsorptive properties of the powder. The values of ±(δ d/d) for various types of powder lie between ± 0·05 per cent and ± 0·4 percent. It can be concluded from this investigation that the extent of the lattice defects caused by the grinding of the powder is an additional parameter characterizing the properties of commercial lead powders.     

Preparation of three-dimensional ordered macroporous SiCN ceramic using sacrificing template method

Three-dimensional (3D) long range well ordered macroporous SiCN ceramics were prepared by infiltrating sacrificial colloidal silica templates with the low molecular weight preceramic polymer, polysilazane. This was followed by a thermal curing step, pyrolysis at 1250 °C in a N₂ atmosphere, and finally the removal of the templates by etching with dilute HF. The produced macroporous SiCN ceramics showed high BET surface areas (pore volume) in the range 455 m²/g (0.31 cm³/g)–250 m²/g (0.16 cm³/g) with the pore sizes of 98–578 nm, which could be tailored by controlling the sizes of the sacrificial silica spheres in the range 112–650 nm. The sphere-inversed macropores were interconnected by 50 ± 30 nm windows and 3–5 nm mesopores embedded in the porous SiCN ceramic frameworks, which resulted in a trimodal pore size distribution. The surface of the achieved porous SiCN ceramic was then modified by Pt–Ru nanoparticle depositing under mild chemical conditions.     

Effect of engine-based thermal aging on surface morphology and performance of Lean NOx Traps

A small single-cylinder diesel engine is used to thermally age model (Pt + Rh/Ba/γ-Al₂O₃) lean NO_x traps (LNTs) under lean/rich cycling at target temperatures of 600 °C, 700 °C, and 800 °C. During an aging cycle, fuel is injected into the exhaust to achieve reproducible exotherms under lean and rich conditions with the average temperature approximating the target temperature. Aging is performed until the cycle-average NO_x conversion measured at 400 °C is approximately constant. Engine-based NO_x conversion decreased by 42% after 60 cycles at 600 °C, 36% after 76 cycles at 700 °C and 57% after 46 cycles at 800 °C. The catalyst samples were removed and characterized by XRD and using a microreactor that allowed controlled measurements of surface area, precious metal size, NO_x storage, and reaction rates. Three aging mechanisms responsible for the deactivation of LNTs have been identified: (i) loss of dispersion of the precious metals, (ii) phase transitions in the washcoat materials, and (iii) loss of surface area of the storage component and support. These three mechanisms are accelerated when the aging temperature exceeds 850 °C—the γ to δ transition temperature of Al₂O₃. Normalization of rates of NO reacted at 400 °C to total surface area demonstrates the biggest impact on performance stems from surface area losses rather than from precious metal sintering.     

Fabrication of microporous ceramics from ceramic powders of quartz–natural zeolite mixtures

The possibility of producing ceramic powders suitable for the fabrication of microporous filters was investigated through the thermal treatment of the powder mixtures of a high-purity (99.09% SiO₂) quartz and clinoptilolite type of natural zeolite. The quartz and zeolite, mixed in the ratio of 3:1 by weight, was wet ground in a ball mill, the powder was sieved on a 90-μm screen, and the undersize was dried and sintered in the powder form at the temperatures of 1000, 1100 and 1200 °C for 7 h in an air furnace. The powder sinter products were deagglomerated by gentle crushing in an agate mortar and then characterized by phase composition, density, and specific surface area measurements. The added zeolite facilitated the transformation of quartz to cristobalite. The phase transformation of quartz to cristobalite first appeared at around 1000 °C, and, at 1200 °C, led to a ceramic powder sufficiently high in cristobalite content for the fabrication of the microporous ceramic bodies. Re-sintering at 1200 °C of the pressed forms of the ceramic powder resulted in microporous (0.5–3 μm) ceramics with a high porosity of 48.5%, and a three-point bend strength of 140 kg/cm². The ceramics obtained may have potential for filter applications.     

Highly porous macro- and micro-cellular ceramics from a polysilazane precursor

Micro- and macro-cellular SiCN and SiOCN foams were produced via two different routes by using a polysilazane preceramic polymer. In the first route, a mixture of partially cross-linked polysilazane and poly(methylmetacrylate) microspheres, used as sacrificial fillers, was warm pressed and subsequently pyrolyzed to create micro-cellular foams. In the second route, liquid polysilazane was mixed with a physical blowing agent and the blend was cured and pyrolyzed, leading to the formation of macro-cellular ceramics in a one-step process. Ceramic components of different morphology and characteristics, depending on the processing method adopted, were fabricated. The foams had a mostly interconnected porosity ranging from 60 to 80 vol% and possessing a compressive strength in the range 1–11 MPa. Some oxygen contamination was found in the foams obtained using the sacrificial fillers, probably because of the adsorbed humidity on their surface. The polymer derived ceramic (PDC) route is an efficient and cost effective way to produce SiCN-based foams possessing tailored pore architecture and properties suitable for high temperature applications.     

A study of the deactivation by sulfur and regeneration of a model NSR Pt/Ba/Al2O3 catalyst

The deactivation by sulfur and regeneration of a model Pt/Ba/Al₂O₃ NO_x trap catalyst is studied by hydrogen temperature programmed reduction (TPR), X-ray diffraction (XRD), and NO_x storage capacity measurements. The TPR profile of the sulfated catalyst in lean conditions at 400 °C reveals three main peaks corresponding to aluminum sulfates (550 °C), “surface” barium sulfates (650 °C) and “bulk” barium sulfates (750 °C). Platinum plays a role in the reduction of the two former types of sulfates while the reduction of “bulk” barium sulfates is not influenced by the metallic phase. The thermal treatment of the sulfated catalyst in oxidizing conditions until 800 °C leads to a stabilization of sulfates which become less reducible. Stable barium sulfides are formed during the regeneration under hydrogen at 800 °C. However, the presence of carbon dioxide and water in the rich mixture allows eliminating more or less sulfides and sulfates, depending on the temperature and time. The regeneration in the former mixture at 650 °C leads to the total recovery of the NO_x storage capacity even if “bulk” barium sulfates are still present on the catalyst.     

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.     

Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.     

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.     

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.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.