Electron microscopic studies on magnetosomes in magnetotactic bacteria have revealed much information on their composition, structure, and even the formation of their mineral phase. The mineral phases of the magnetosomes are of two general types: iron oxides and iron sulfides. Iron oxide-type magnetosomes contain particles of the ferrimagnetic mineral magnetite (Fe3O4) while the iron sulfide-type contain ferrimagnetic greigite (Fe3S4), greigite and non-magnetic pyrite (FeS2), or possibly ferrimagnetic pyrrhotite (Fe7S8). Regardless of their composition, the crystalline particles in magnetosomes have a narrow size range: approximately 35 to 120 nm. Magnetite crystals in this size range are single-magnetic-domains and confer a permanent magnetic dipole moment to the cell. The single-domain size range for greigite is not known but is probably similar to that for magnetite. The morphology of the particles in the bacterial magnetosomes appears to be species-specific. Morphologies of magnetite crystals in different species of magnetotactic bacteria include cubo-octahedra, parallelepipedal (truncated hexahedral or octahedral prisms), and tooth- or bullet-shaped (anisotropic). Morphologies of greigite particles include cubo-octahedra and rectangular prismatic. The greigite-pyrite particles are generally pleomorphic with no consistent crystalline morphology. A membrane has been shown to surround the particles in some organisms and may be involved in the formation of the crystalline phase while also providing physical constraints on the size and the shape of the crystal. These results clearly indicate that the biomineralization process involved in the bacterial magnetosome, a good example of a self-assembled structure on a nanometer scale, is highly controlled by the organism.

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Two commercial oxide dispersi?n ~trengthened alumina-forming FeCrAl alloys, Inco alloy MA956 and Kant~al alloy APM, were studted m order to look at the effect of reactive elements on their oxidation b~ha~to~r. MA956 has ~ d~strihution of Y 20 3 -Ah03 particles, while APM has a ZrO:rAh03 dtstnbutton. Isothermal oxtdatton at lOOOoc and 1200oc showed a reduced oxidation rate for both alloys compared to that of ~n und<:>ped FeC~Al alloy. In short-term cyclic tests at l200°C, both alloys exhibited excellent scale adheston. Usmg scan~mg transmission electron microscopy with X-ray energy dispersive spectroscopy~ both Y and Zr, r~spe~ttvely, were found to segregate to the oxide grain boundaries and the ~etal-scale. mterface after oxtdatton at lOOOoC and l200°C. These experimental observations are dtscussed wtth regard to a new theory to explain the reactive element effect.

A quantitative study of space charge solute segregation at grain boundaries in TiO, is conducted, using a new STEM method for the measurement of aliovalent solute accumulation. It is shown that the electrostatic potential at grain boundaries can be varied in sign and magnitude with doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor-doped compositions for samples annealed in air. The experimental results closely fit the space charge model in Part I. Space charge solute segregation is found even in defect regimes of high electron concentration. Approximately one in ten grain boundaries are "special" in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with significant amounts of segregation, clear differences in potential also exist. From the potential determined in acceptor-and donor-doped compositions, the Frenkel energy (assumed to be lower than the Schottky energy in TiO,) can be separated into its individual terms. An average value for the titanium vacancy formation energy of gVn = 2.4 eV and an upper limit to the titanium interstitial formation energy of g , = 2.6 eV are obtained.while the solubility of Nb,O, is reported to be 0.7-3 m~l%'-~ in the temperature range used in this study.Another major distinction between this and previous studies is the use of a highly quantitative method for measuring aliovalent solute segregation. We have developed a scanning transmission electron microscope (STEM) method for accurately measuring the total accumulation of solute at an interface. From the net amount of acceptor or donor segregation as a function of composition and temperature, the relationship between the electrostatic potential and the lattice defect structure has been systematically studied. These results are reported here, and are quantitatively compared with the pxedictions of the model in Part I. Experimental Procedure ( I ) Sample PreparationPowders were coprecipitated from aqueous solutions of TiCI, (Johnson Matthey 99.999%) to which NbCl, (Puratronic 99.999%), AIC13.6H,0 (Puratronic 99.9995%), or Ga(NO,), (Puratronic 99.999%) was added in the desired concentrations. Polyethylene or Teflon labware and 18-MCl deionized water was used throughout powder processing to minimize impurities. A number of additional precautions, discussed in greater detail in Ref. 6, were necessary because of the high volatility of TiCI,, its highly exothermic reaction with water, and the hygroscopic and/or reactive nature of the dopant salts. The basic process' consists of first preparing an aqueous solution of TiCI, by mixing the chloride with water and ice. The dopant salts were dissolved into this solution and homogenized by stirring. To this colorless, precipitate-free solution, a 1 : 1 solution of NH,OH:H,O was slowly added while stimng to precipitate the metal hydroxides, reaching a solution pH of about 10. The precipitate was allowed to digest while stirring for at least 1 h, and then thoroughly washed by repeated dilution...

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