Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

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.     

Hydrothermal synthesis, characterization and structure refinement of chlorate- and perchlorate-sodalite

Chlorate- and perchlorate-sodalites were synthesized hydrothermally in the temperature range of 160°C to 500°C. IR absorption bands indicate the enclathration of NaClO₃ (624 cm⁻¹) and NaClO₄ (624 cm⁻¹ and 2050 cm⁻¹) in the sodalite cages. The thermal decomposition has been characterized by simultaneous thermal analysis (TG, DTG, DTA) and high temperature X-ray powder diffraction. The total collapse of the sodalite framework structure and the formation of nepheline could be observed at 750°C and 1100°C for chlorate-sodalite and perchlorate-sodalite, respectively.

The crystal structure has been determined for NaClO₄-sodalite showing cubic symmetry (a₀=9.071 Å, SG P 3n) and complete ordering of Si and Al in the silicate framework; the Si-O-Al angle is 146.7°. The Cl atoms of the enclathrated perchlorate are located at the central positions of the sodalite cages. For the oxygen atoms of the Cl₄⁻ anions the structure refinement led to orientationally disordered sites having a close resemblance to the well-known O(2) positions of basic sodalite Na₈[AlSiO₄]₆(OH)₂·2 H₂O.     

Nitridation of silicon powder studied by XRD, Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900–1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and ²⁹Si MAS NMR. An initial mass gain of about 12% at 1250–1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si₂N₂O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of - and β-Si₃N₄, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400–1475 °C, the ²⁹Si MAS NMR chemical shift of the elemental Si changes progressively from about −80 ppm to −70 ppm, in tandem with the growth of the Si₃N₄ resonance at about −48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. ©     

ß-sialon ceramics with TiN particle inclusions

Fully dense composites of 0–30 wt% discrete TiN particles distributed in a ß-sialon matrix of overall composition Si_5·5Al_0·5O_0·5N_7·5 have been prepared by hot isostatic pressing at 1650 and 1750°C. Pressureless sintering at 1775°C gave materials with an open porosity. Typical sizes of the TiN particles were 1–3 μm, and no intergranular glassy phase was observed in the prepared materials. The grain size of ß-sialon was below 1 μm in the materials HIPed at 1650°C, and 1–2 μm at 1750°C. The Vickers hardness was fairly constant for the TiN-ß-sialon composites with up to 15 wt% TiN added: H_v10 around 17·5 GPa for materials HIPed at 1650° and around 17 GPa at 1750°C, whereas at higher TiN contents the hardness decreased to around 16 GPa. The indentation fracture toughness of the ß-sialon ceramic increased approximatively from 3 to 4 MPam^1/2 at an addition of 15 wt% TiN particulates. The fracture toughness could be further increased to 5 MPam^1/2 by addition of small amounts of Y₂O₃ and A1N to a ß-sialon composite with 30 wt% TiN.     

Electrical conductance of KI in anhydrous acetonitrile

The electrical conductivity of KI solutions in anhydrous acetonitrile has been determined at 0, 25 and 35°C in the concentration range 0·9– 600 × 10⁻⁴ mole/l. The values of Λ₀, K and a calculated from the results are, respectively: 145·9 mho/cm, 0·95 × 10 ⁻² and 1·72 Å at 0°C; 186·2 mho/cm, 8·98 × 10⁻² and 4·6 Å at 25°C; and 204·8 mho/cm, 5·17 × 10⁻² and 3·5 Å at 35°C. The phoreograms at all the three temperatures are catabatic at lower concentration, but become anabatic at 0·017, 0·022 and 0·024 respectively, at 0, 25 and 35°C.     

Fabrication and conductivity of a new compound Ca2Cr2O5

A new brownmillerite-related compound. Ca₂Cr₂O₅, has been prepared. It has been indexed according to an orthorhombic lattice a = 5·750 Å, b = 14·398 Å and c = 5·483 Å. A series of experiments was performed in order to find the appropriate firing temperature. The total conductivity was measured by a four-point method in the range of 690–911°C. Impedance spectroscopy was also employed in the temperature range 343–785°C. Conductivity measurements at different oxygen pressures at 500°C suggest that Ca₂Cr₂O₅ is a predominantly ionic conductor at P_o2 = 1–10⁻²atm.     

Synthesis of single-phase Ti3SiC2 powder

In this study, free 2Ti/2Si/3TiC powder mixture was heated at high temperatures in vacuum, in order to reveal the possibility for the synthesis of high Ti₃SiC₂ content powder. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the evaluation of phase identities and the morphology of the powder after different treatments. Results showed that almost single phase Ti₃SiC₂ powder (99.3 wt.%) can be synthesized by heat treatment with free 2Ti/2Si/3TiC powders in vacuum at 1210°C for about 3 h. The nucleation and growth of Ti₃SiC₂ within TiC particles was observed. The typical appearance of the formed Ti₃SiC₂ is equiaxed with particle size of 2–4 μm. Effects of temperature and heating time on the morphology and the particle sizes of the synthesized Ti₃SiC₂ powders are not obvious.     

Synthesis of platelike CaTiO3 particles by a topochemical microcrystal conversion method and fabrication of textured microwave dielectric ceramics

Platelike CaTiO₃ particles with an orthorhombic perovskite structure have been synthesized by topochemical microcrystal conversion (TMC) from platelike precursor particles of the layer-structured CaBi₄Ti₄O₁₅ at 950 °C. The CaTiO₃ particles inherited and retained the shape of the precursor particles with a thickness of approximately 0.3 μm, and a width of 2–6 μm. XRD analysis showed that in the TMC reaction, the crystallographic {0 0 1} plane of CaBi₄Ti₄O₁₅ is converted into the {1 0 0} plane of CaTiO₃. Using the platelike CaTiO₃ particles as templates in the templated grain growth method, dense {1 0 0} grain-oriented CaTiO₃ ceramics having a {1 0 0} orientation could be fabricated at sintering temperatures between 1350 and 1500 °C. The maximum orientation factor reached 99.7% at 10% of template. It was found that texturing improves microwave dielectric low-loss properties, providing a 1.55 times higher Q_f value of 9310 GHz in textured ceramics compared to that of 6005 GHz in non-textured ceramics.     

Elaboration and characterization of a refractory based on Algerian kaolin

A refractory material was elaborated from kaolin extracted from the region of Djebel Debbagh (Algeria). Kaolin grog was obtained by calcination at a temperature of 1350 °C during 1 h. It was used as aggregates with granulometric distribution composed of fine fraction (mean grain size: 100–250 μm) and coarse fraction (mean grain size: 1000–2500 μm). Crude kaolin (size < 75 μm) was also used as a binder with an amount representing 15% of the dry material. After a 9.28% moistening and a rotting of 1 day, cylindrical samples were shaped by uniaxial pressure at 80 MPa. The samples were submitted to a natural drying during 24 h, a stoving at 100 °C and a calcination at 600 °C during 1 h. They were fired at high temperatures between 1250 and 1450 °C.

An X-ray diffraction (XRD) analysis showed that the refractory samples are composed of mullite and silica. Silica is a mixture of a vitreous phase and cristobalite at 1300, 1350 and 1400 °C and becomes completely amorphous when the samples are fired at higher temperature (1450 °C). The sample porosity is about 30%. The mechanical tests carried out as a function of temperature revealed different behaviours of the material. From the ambient up to 600 °C, the refractory behaviour is pseudo-plastic caused by micro-cracking. Between 700 and 900 °C, the samples become more rigid. At 1000 °C, the material exhibits a visco-plastic behaviour. The amorphous phase governs the sample properties variation with temperature increasing. Its content varies between 28% and 34% according to the firing temperature. Thermal shock tests realized in water showed that the refractory samples present good thermal shock resistance.     

Conventional and microwave hydrothermal synthesis of zeolite ZSM-5 from the cupola slag

ZSM-5 type zeolites have been prepared from cupola slag waste using both conventional hydrothermal and microwave syntheses at 130–200 °C. The ZSM-5 was synthesized by conventional heating by taking advantage of the high silica content of cupola slags. Microwave heating increased the rate of ZSM-5 formation by 4 times at 150 °C compared with conventional heating. The Si/Al ratio of the ZSM-5 produced by the conventional heating and the microwave crystallization were similar 28 and 29, respectively. The conventional-heating produced ZSM-5 particles 3 μm in diameter, while, microwave-heating produced smaller ZSM-5 particles only 0.3 μm in size.     

Preparation of spherical yttrium oxide powders using emulsion evaporation

Water-in-oil type emulsions were used to prepare yttrium oxide powders by evaporation of the yttrium-containing aqueous phase in a hot oil bath. Emulsions were characterized with respect to emulsion type, droplet size distribution and stability. Y₂O₃ powders obtained by this method consisted of small (1–3 μm) spherical granules made up of 0·1 μm crystallites. The effects on the powder of emulsifier concentration, yttrium ion concentration, evaporation temperature and atmosphere were studied.     

The influence of reaction parameters on the free Si and C contents in the synthesis of nano-sized SiC

Uniform nano-sized beta-silicon carbide (β-SiC) powder was synthesized from the reaction of silicon (Si) and carbon black (C). Mixed Si and C-black powder were pressed into pellets and the influence of four parameters, temperature (1250, 1300 and 1350 °C), heating rate (20 and 50 °C/min), soaking time (1 and 3 h) and atmosphere (vacuum and argon), were tested. It was found that higher temperatures, higher heating rates and longer soaking times in a vacuum system lead to lower free Si content in the SiC powder created. Temperature was the parameter with the greatest influence on the Si content of the SiC powder. This study also found that the Si–C reaction occurs through gas–solid (SiO–C) and solid–solid (Si–C) reactions that occur simultaneously.     

Structure of a cyclopropane sorption complex of dehydrated fully Mn-exchanged zeolite X

The structure of a cyclopropane sorption complex of dehydrated fully Mn²⁺-exchanged zeolite X, Mn₄₆Si₁₀₀Al₉₂O₃₈₄ · 30C₃H₆ (a=24.690(4) Å), has been determined by single-crystal X-ray diffraction techniques in the cubic space group at 21(1)°C. The crystal was prepared by ion exchange in a flowing stream of 0.05 M aqueous Mn(NO₃)₂ for three days, followed by dehydration at 460°C and 2×10⁻⁶ Torr for two days, and exposure to 100 Torr of cyclopropane gas at 21(1)°C. The structure was determined in this atmosphere and was refined to the final error indices R₁=0.065 and R₂=0.071 with 509 reflections for which I>3σ (I). In this structure, Mn²⁺ ions are located at two crystallographic sites. Sixteen Mn²⁺ ions fill the octahedral site I at the centers of the hexagonal prisms (Mn–O=2.290(9) Å). The remaining 30 Mn²⁺ ions are at site II; each extends 0.41 Å into the supercage (an increase of 0.27 Å upon C₃H₆ sorption as compared to fully dehydrated Mn₄₆Si₁₀₀Al₉₂O₃₈₄) where it coordinates to three trigonally arranged framework oxygens at 2.148(8) Å and complexes weakly and facially to a cyclopropane molecule by a primarily quadrupolar interaction. The carbon atoms of each cyclopropane molecule are equivalent and equidistant from its Mn²⁺ ion (Mn–C=2.95(9) Å). Because of high thermal motion, the C–C bond length is inaccurately determined; the value found, 1.21(8) Å, is too small.     

Development of stable supports consisting of SiC---Si composite for high temperature combustion catalysts

SiC-Si composite, that is stable in oxidizing atmosphere at 1300°C and has thermal shock resistance, was prepared from a powder mixture of porous β-SiC, which was prepared from rice hulls, and Si metal. To use an SiC-Si composite as a structural support for a high temperature combustion catalyst, the foaming SiC-Si composite form with continuous bubbles was prepared from foaming SiC form and the mixture of the porous β-SiC and Si metal. The foaming SiC form was prepared from the foaming polyurethane form and a β-SiC fine particles. The β-SiC fine particles having an average diameter of 0.3 μm was coated on the foaming polyurethane form. The polyurethane part of the form was burned out and the coating β-SiC was sintered to form the foaming SiC form. The SiC form was coated on the porous SiC and Si metal powder mixtures and was heated at 1500°C in argon to prepare the foaming SiC-Si composite. The foaming composite was stable in an oxidizing atmosphere at 1300°C and was highly resistance to thermal shock. The compression stress of the foaming SiC-Si composite form (175 kg/cm²was about twice that of the a-axis of honeycomb-shaped cordierite (> 85 kg/cm²).     

Dense ceramics of BaTiO3 produced from powders prepared by a chemical process

A sintering optimization of barium titanate ceramics from fine-grained and homogeneous reproducible powders obtained by the citric process is presented. Different sintering parameters are studied: heating rates, final temperature, dwelling times at this final temperature, and influence of the powder deagglomeration step. The sintering is followed by dilatometric measurements. The ceramics obtained by sintering at 1230 or 1300 °C are free of barium carbonate, the residual carbon content being estimated at about 400 ppm in the surface layer. They exhibit a grain size close to 1 μm, a structure in which the cubic and tetragonal phases coexist, and a density of about 96% of the theoretical density. Their permittivity and loss factor are respectively about 5000 and 2.5 × 10⁻² at 25 °C.     

Synthesis and characterization of gadolinium phosphate neutron absorber

Hydrated gadolinium phosphate (GdPO₄·1H₂O) was synthesized by reacting high purity dissolved salts (gadolinium nitrates or chlorides) with phosphoric acid. The hydrated powders were shown to be extremely insoluble in water with a K_sp measured to be between 2.07 E-14 and 4.76 E-13. Calcination to between 800 and 1000 °C resulted in the formation of GdPO₄ in a monazite (monoclinic) crystal structure. This was correlated with the first exothermic differential thermal analysis (DTA) peak (864.9–883.4 °C). The DTA also showed small peaks in the 1200–1250 °C range, that could be associated with a change from the monazite (monoclinic) crystal structure to the xenotime (tetragonal) crystal structure. However, calcination of a sample to 1400 °C, followed by relatively rapid cooling and XRD, showed the structure was still monazite (monoclinic). DTA results showed a melting point at 1899–1920 °C (endothermic peak). It was therefore concluded that the melting point probably was the melting of the monazite (monoclinic) phase, but may have been xenotime if a phase change at 1200–1250 °C was reversible and very rapid. The higher part of the melting range was achieved with material derived using the slightly higher purity nitrate salt. The results show that GdPO₄ is an excellent candidate for a chemically stable, water-insoluble neutron absorber for inclusion in spent nuclear fuel canisters.     

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.     

Aerosol measurement by laser doppler spectroscopy—II. Operational limits, effects of polydispersity, and applications

The theoretical basis and the results of a computer simulation are presented which describe the operational limits of size and concentration for aerosol sizing by laser Doppler spectroscopy LDS,. This analysis suggests that a state of the art LDS system has the capability of sizing 0·03 μm diameter particles when the number concentration is 10⁸ cm⁻³ or greater and 0·2 μm diameter for coocentrations as low as 100 particles cm⁻³. An evaluation of the effect on the laser Doppler spectroscopy measurements of a polydisperse aerosol having a log normal size distribution is presented and methods for combining these measurements with other averaged measurements to determine both count median diameter (CMD) and geometric standard deviation (δ_g) are proposed. For aerosols having log normal distributions with 0·3 < CMD < 3 μm and 1·0 < δ_g < 2·0, laser Doppler spectroscopy is able to measure the surface area median diameter within ± 15 per cent, independent of polydispersity. Applications of LDS to aerosol sizing are evaluated and its advantages and disadvantages relative to other sizing methods are discussed.     

Synthesis of nanocrystalline 8 mol% yttria stabilized zirconia powder from sucrose derived organic precursors

An organic precursor synthesis of 8 mol% yttria stabilized zirconia (YSZ) powder from Zr–Y composite nitrate solution and sucrose has been studied. Oxidation of sucrose in Zr–Y composite nitrate solution containing excess nitric acid in situ generates hydroxy carboxylic acids that forms a white sol which showed peaks at 1640 cm⁻¹ and 1363 cm⁻¹ in IR spectrum corresponding to hydroxy carboxylic acid complexes of Zr and Y. Precursor mass obtained by drying the sol on calcinations at 600 °C produced loosely agglomerated particles of cubic YSZ. Deagglomerated YSZ contain submicron particles with D₅₀ value of 0.5 μm and the particles are aggregates of nanocrystallites of nearly 10 nm size. Compacts prepared by pressing the YSZ powder sintered to 96.7% TD at 1450 °C. The sintered YSZ ceramic showed an average grain size of 2.2 μm.