Developments in the control of fine particulate air emissions

Five commercial barium titanate powders were subjected to various surface decontamination treatments: calcination at 500 °C, followed by leaching either in pure water or acetate buffer solution. They were then de-agglomerated by ultrasonic treatment and finally characterised by XRD, TGA, XPS, EGA, pyrolysis/FT-IR, BET specific surface area and particle size analysis. The main contaminating elements in raw powders were carbon and sulphur, present as barium carbonate (between 0.7 and 1.2wt%) and barium sulphate (about 0.5 wt%). Barium sulphate is thermally stable up to 1400 °C and is not affected by the treatments. Barium carbonate decomposes in two steps, between 500 and 900 °C and 1200 and 1400 °C by reaction with TiO2 and BaTiO3 to release CO2. Carbonation is slightly reduced by calcination, but subsequent water cleaning promotes re-carbonation. Acid cleaning substantially decreases the BaCO3 content, down to 0.07–0.7 wt%. The remaining quantity corresponds to the most stable form which decomposes near 1200–1400 °C and is still partially present at the surface of the particles. Forming of ceramics green bodies by slip casting of aqueous suspensions stabilised by ammonium poly (acrylate) was then investigated. The dispersing solution compositions have been optimised for each powder and treatment, in order to minimise the liquid/solid ratio of the slip. In all cases, the properties of the slips and of the cast specimens have been improved by the cleaning treatments: higher solid volume fractions in the slip up to 60% for the best case and higher green densities. In particular green densities as high as 70% have been obtained which is very close to the theoretical maximum packing density of log-normally polydispersed spheres. This enhancement can be ascribed to the beneficial effect of decarbonation on the deagglomeration of the powders.

Surface decontamination treatments of undoped BaTiO3—part I: Powder and green body properties

The role of liquid phase in the enhancement of the PTCR (positive temperature coefficient of resistance) effect in (Ba 0.7 Sr 0.3 )TiO 3 (BST) with the addition of AST (4Al 2 O 3 . 9SiO 2 .3TiO 2 ) is investigated in this paper. The AST-BST samples were characterized with optical microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and impedance spectroscopy. Microscopic observations showed that slower cooling might facilitate the precipitation of the (Ba,Sr)TiO 3 phase from the liquid phase on matrix grains since the amount of liquid phase was reduced with a decreasing cooling rate. Impedance spectroscopy indicated that this variation accompanied the change in the intrinsic properties of grain boundaries, which could not be explained by well-known oxidation effects. With the aid of a brick-layer model and high-resolution transmission electron microscopy (HRTEM), it appeared that the change in electrical characteristics of grain boundaries with decreasing cooling rate originated from the precipitation of (Ba,Sr)TiO 3 . Finally, the effect of precipitated (Ba,Sr)TiO 3 on the PTCR characteristics is discussed in terms of the acceptor-state density and the polarization state at grain boundaries.

Role of Liquid Phase in PTCR Characteristics of (Ba0.7Sr0.3)TiO3 Ceramics

Porous and fine-grained (Ba,Sr)TiO3 ceramics were fabricated by adding partially oxidized Ti powders. During heating and cooling, the electrical resistance was measured in air, O2, N2, and H2 atmospheres and in H2 followed by air atmosphere. The effect of atmosphere on the PTCR characteristics and the role of oxygen on the grain-boundaries in the PTCR characteristics were investigated. The porous (Ba,Sr)TiO3 ceramics showed a large PTCR effect, which was strongly sensitive to atmosphere. This effect was increased in oxidizing atmosphere mainly because of an increase in potential barrier height whereas decreased in reducing atmosphere mainly because of a decrease in potential barrier height.

Effect of atmosphere on the PTCR characteristics of porous (Ba,Sr)TiO3 ceramics

A site deficient Bi doped BaTiO3 ceramic with general formula Ba1-xBi2x/3TiO3 (x = 0.00, 0.01, 0.025) is prepared by solid state reaction route. The phase formation and structural property of all compositions are studied by X-Ray Diffraction pattern. The pattern reports single phase tetragonal crystal system with space group of P4mm. The XRD study also reveals that bismuth (Bi) replaces “A” site (Ba) of the BaTiO3 pervoskite. The surface morphology of the sintered pellets is studied by scanning electron microscopy which shows a decrease in grain size with an increase in Bi concentration. The temperature and frequency dependent dielectric behaviors of the compositions are studied to show the effect of Bi ion on the “A” (Ba) site of BaTiO3 perovskite. The dielectric constant decreases and transition temperature increases with increase in Bi concentration. Substitution of Bi ion induces diffuse ferroelectric behavior and the degree of diffuseness increases with increase in doping concentration. The ferroelectric behavior is also confirmed by the P-E loop study.

Relaxor ferroelectric behavior of “A” site deficient Bismuth doped Barium Titanate ceramic

Increased demands are being placed on air filtration systems. The particular application will determine whether a baghouse or cartridge type is best. Baghouse and cartridge dust collectors both have their place in modern air filtering systems. Baghouses have been in use much longer, but cartridge types offer significant advantages for particular applications. The task facing the site engineer is to match the requirements of the specific application with the inherent characteristics of the dust collector. This article presents basic information about both types of dust collectors that can help provide the best solution to that problem.

G. Kelley

Papers

Baghouse and cartridge dust collectors: A comparison