Showing papers on "Bismuth published in 1999"

Lanthanum-substituted bismuth titanate for use in non-volatile memories

Abstract: Non-volatile memory devices are so named because they retain information when power is interrupted; thus they are important computer components. In this context, there has been considerable recent interest1,2 in developing non-volatile memories that use ferroelectric thin films—‘ferroelectric random access memories’, or FRAMs—in which information is stored in the polarization state of the ferroelectric material. To realize a practical FRAM, the thin films should satisfy the following criteria: compatibility with existing dynamic random access memory technologies, large remnant polarization (Pr) and reliable polarization-cycling characteristics. Early work focused on lead zirconate titanate (PZT) but, when films of this material were grown on metal electrodes, they generally suffered from a reduction of Pr (‘fatigue’) with polarity switching. Strontium bismuth tantalate (SBT) and related oxides have been proposed to overcome the fatigue problem3, but such materials have other shortcomings, such as a high deposition temperature. Here we show that lanthanum-substituted bismuth titanate thin films provide a promising alternative for FRAM applications. The films are fatigue-free on metal electrodes, they can be deposited at temperatures of ∼650 °C and their values of Pr are larger than those of the SBT films.

Bismuth based oxide electrolytes— structure and ionic conductivity

Abstract: Bismuth oxide systems exhibit high oxide ion conductivity and have been proposed as good electrolyte materials for applications such as solid oxide fuel cells and oxygen sensors. However, due to their instability under conditions of low oxygen partial pressures there has been difficulty in developing these materials as alternative electrolyte materials compared to the state-of-the-art cubic stabilised zirconia electrolyte. Bismuth oxide and doped bismuth oxide systems exhibit a complex array of structures and properties depending upon the dopant concentration, temperature and atmosphere. In this paper we comprehensively review the structures, thermal expansion, phase transitions, electrical conductivity and stability of bismuth oxide and doped bismuth oxide systems. ©

Large Magnetoresistance of Electrodeposited Single-Crystal Bismuth Thin Films

Abstract: Single-crystal bismuth thin films 1 to 20 micrometers thick were fabricated by electrodeposition and suitable annealing. Magnetoresistance up to 250 percent at 300 kelvin and 380,000 percent at 5 kelvin as well as clean Shubnikov-de Haas oscillations were observed, indicative of the high quality of these films. A hybrid structure was also made that showed a large magnetoresistive effect of 30 percent at 200 oersted and a field sensitivity of 0.2 percent magnetoresistance per oersted at room temperature.

Schottky barrier heights of tantalum oxide, barium strontium titanate, lead titanate, and strontium bismuth tantalate

Abstract: The Schottky barrier heights of various metals on the high permitivity oxides tantalum pentoxide, barium strontium titanate, lead zirconate titanate, and strontium bismuth tantalate have been calculated as a function of the metal work function. It is found that these oxides have a dimensionless Schottky barrier pinning factor S of 0.28–0.4 and not close to 1 because S is controlled by Ti–O-type bonds not Sr–O-type bonds, as assumed in earlier work. The band offsets on silicon are asymmetric with a much smaller offset at the conduction band, so that Ta2O5 and barium strontium titanate are relatively poor barriers to electrons on Si.

First-principles investigation of ferromagnetism and ferroelectricity in bismuth manganite

Abstract: We present results of local spin density approximation (LSDA) pseudopotential calculations for the perovskite structure oxide, bismuth manganite (BiMnO3). The origin of the differences between bismuth manganite and other perovskite manganites is determined by first calculating total energies and band structures of the high symmetry cubic phase, then sequentially lowering the magnetic and structural symmetry. Our results indicate that covalent bonding between bismuth cations and oxygen anions stabilizes different magnetic and structural phases compared with the rare earth manganites. This is consistent with recent experimental results showing enhancement of charge ordering in doped bismuth manganite.

Processing and Characterization of Single-Crystalline Ultrafine Bismuth Nanowires

Abstract: By pressure injecting Bi liquid melt into the nanochannels of an anodic alumina template, we have successfully fabricated Bi nanowire arrays with ultrafine wire diameters and extremely high wire packing densities. Free-standing Bi nanowires with controlled wire diameters and large aspect ratios (length/diameter) were also obtained by subsequent etching of the alumina template. Various techniques such as SEM, TEM, AFM, EFM, HREM, and XRD have been used to investigate the physical characteristics of these nanowires. The Bi nanowires were found to be dense and continuous and had a uniform diameter throughout the length of the wires. Individual Bi nanowires were shown to be single crystals, and all the wires in an array were highly oriented. An interesting metastable phase of Bi was also observed, which can be attributed to a lattice stress-induced high-pressure phase of Bi formed inside the porous anodic alumina template.

Thermoelectric power of bismuth nanowires

Abstract: We report here measurements of the thermoelectric power and longitudinal magneto-Seebeck coefficient of 200 nm diameter single-crystal bismuth nanowires. Nanowires of pure Bi and of n-type-doped Bi (with Te at about $5\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ were measured. The wires are imbedded in porous anodic alumina. The data are taken on arrays of wires connected in parallel, at temperatures from 8 to 300 K, and, between 10 and 80 K, in magnetic fields from 0 to 5 Tesla. It has been theoretically calculated that bismuth nanowires should have a strongly increased thermoelectric figure of merit over bulk Bi, when the diameter is decreased below about 10 nm. The nanowires in this study were selected because they are easier to prepare and handle. The temperature-dependent thermopower data are consistent with the partial electron and hole thermopower values calculated using the carrier Fermi energies obtained from Shubnikov--de Haas oscillations on the same samples.

Electrodeposited ceramic single crystals

Abstract: Single-crystal films are essential for devices because the intrinsic properties of the material, rather than its grain boundaries, can be exploited. Cubic bismuth oxide has the highest known oxide ion mobility, which makes it useful for fuel cells and sensors, but it is normally only stable from 729° to 825°C. The material has not been previously observed at room temperature. Single-crystal films of the high-temperature cubic polymorph of bismuth oxide were epitaxially electrodeposited from an aqueous solution onto single-crystal gold substrates. The 35.4 percent lattice mismatch was accommodated by forming coincidence lattices in which the bismuth oxide film was rotated in relation to the gold substrate. These results provide a method for producing other nonequilibrium phases that cannot be accessed by traditional thermal processing.

Electrochemical degradation of phenol in aqueous solution on bismuth doped lead dioxide : a comparison of the activities of various electrode formulations

Abstract: This paper describes the development of electrochemical processes for the oxidative degradation of toxic organic chemicals in waste waters. Doped bismuth lead dioxide anodes have been tested by the kinetic study of phenol anodic oxidation in aqueous solution. The main products during oxidative degradation of phenol are 1,4- benzoquinone, maleic acid and carbon dioxide. Several deposits of Bi2O5–PbO2 on Ti/(IrO2–Ta2O5) substrates have been prepared by anodic oxidation of Pb2+ and Bi3+ in aqueous solutions containing perchloric acid to increase the solubility of bismuth. To study the effect of perchlorate ions, the efficiency of the PbO2 deposit prepared from lead nitrate in an aqueous solution (pure PbO2) was compared with that of a deposit prepared from perchloric acid solution (perchlorate doped PbO2). Although the phenol is oxidized at the same rate on the two deposits, the charge corresponding to the total elimination of 1,4-benzoquinone is three times higher for perchlorate doped PbO2 than for pure PbO2. Phenol degradation is more efficiently carried out on a PbO2 anode doped with perchlorate and with bismuth than on the same electrode doped only with perchlorate. Among the electrodes tested in this work, the pure PbO2 anode is the most efficient for phenol degradation. It is assumed that certain active sites on the anode occupied by perchlorate ions do not participate in the transfer of oxygen atoms and that for the PbO2 electrode doped with bismuth, oxygen evolution is favoured to the detriment of oxygen atom transfer.

Infrared reflective color pigment

Abstract: The present invention provides new solid solutions having a corundum-hematite crystalline structure which are useful as inorganic color pigments. Solid solutions according to the present invention include a host component having a corundum-hematite crystalline structure which contains as guest components one or more elements from the group consisting of aluminum, antimony, bismuth, boron, chrome, cobalt, gallium, indium, iron, lanthanum, lithium, magnesium, manganese, molybdenum, neodymium, nickel, niobium, silicon, tin, titanium, vanadium, and zinc. Solid solutions according to the present invention are formed by thoroughly mixing compounds, usually metal oxides or precursors thereof, which contain the host and guest components and then calcining the compounds to form the solid solutions having the corundum-hematite crystalline structure. Some of the new solid solutions according to the present invention exhibit relatively low Y CIE tri-stimulus values and relatively high near infrared reflectance.

Properties of ternary Sn-Ag-Bi solder alloys: Part II - Wettability and mechanical properties analyses

Abstract: Bismuth additions of 1% to 10% were made to the 96.5Sn-3.5Ag (wt.%) alloy in a study to develop a Sn-Ag-Bi ternary composition. Thermal properties and microstructural analyses of selected alloy compositions were reported in Part I. Wettability and mechanical properties are described in this paper. Contact angle measurements demonstrated that Bi additions improved wetting/spreading performance on Cu; a minimum contact angle of 31±4° was observed with 4.83 wt.% Bi addition. Increasing the Bi content of the ternary alloy raised the Cu/solder/Cu solder joint shear strength to 81 MPa as determined by the ring-and-plug tests. TEM analysis of the 91.84Sn-3.33Ag-4.83Bi composition presented in Part I indicated that the strength improvement was attributed to solid-solution and precipitation strengthening effects by the Bi addition residing in the Sn-rich phase. Microhardness measurements of the Sn-Ag-Bi alloy, as a function of Bi content, reached maximum values of 30 (Knoop, 50 g) and 110 (Knoop, 5 g) for Bi contents greater than approximately 4–5 wt.%.

Lead-free solders

Abstract: Disclosed is a high strength, high fatigue resistance, and high wetting lead-free solder alloy comprising effective amounts of tin, copper, silver, bismuth, indium, and antimony and having a melting temperature between 175-215° C.

Atomically perfect bismuth lines on Si(001)

Abstract: Atomically perfect bismuth lines form on Si(001) by a selective desorption process around the temperature at which most of the bismuth desorbs from bismuth epitaxial layers. The lines are perpendicular to the silicon dimer rows; they are 1 nm wide and can be hundreds of nm long in a flat Si(001) surface. They are utterly free of kinks or other defects. In our proposed model three silicon dimers in the surface are replaced with two Bi dimers, with a rebonded missing dimer defect between them. It is suggested that the lines are straight because of kinetic reasons rather than thermodynamic ones -- they form straight and are unable subsequently to kink.

Preparation and characterization of nanocrystalline powders of bismuth vanadate

Abstract: The influence of mechanical activation on the formation of Bi2VO5.5 bismuth vanadate (BiV) phase, was investigated by ball-milling a stoichiometric mixture of bismuth oxide and vanadium pentoxide. The structural evolution of the desired BN phase, via an intermediate BiVO4,phase, was investigated using X-ray powder diffraction; (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). Milling for 54h. yielded monophasic gamma-BiV powders with an average crystallite size of 30 nm. The electron paramagnetic resonance (EPR) peaks associated with the V4+ ions are stronger and broader in nanocrystalline (n) BN than in the conventionally prepared microcrystalline (m) BN, suggesting theta significant portion of V5+ has been transformed to V4+ during milling. The optical bandgap of n-BiV was found to be higher than that of m-BiV. High density (97% of the theoretical density), fine-grained (average grain-size of 2 tun) ceramics with uniform grain-size distribution could be fabricated using n-BiV powders. These fine-grained ceramics exhibit improved dielectric, pyre and ferroelectric properties. (C) 1999 Elsevier Science S.A. All rights reserved.

Structural Effects on the Oxidation of HCOOH by Bismuth-Modified Pt(111) Electrodes with (100) Monatomic Steps

Abstract: Significant enhancements in the electrocatalytic oxidation of HCOOH are obtained on deliberately stepped platinum surfaces with (111) terraces and (100) monatomic steps when bismuth is deposited on terrace and/or step sites. The bismuth-modified surfaces with higher step densities/narrower terrace widths exhibit higher catalytic activity in the oxidation of HCOOH than surfaces with lower step densities/wider terrace widths, behavior which is opposite to that exhibited by the analogous bare (unmodified) surfaces. For Bi/Pt(544) and Bi/Pt(755), with nine- and six-atom wide (111) terraces, respectively, maximum catalytic behavior occurs when bismuth is deposited only on the steps, suggesting that a critical ensemble (critical size), two atoms wide, enhances the reactive intermediate pathway for the complete oxidation of HCOOH over the poisoning reaction pathway. Moreover, the potential for the onset of oxidation of HCOOH shifts to less positive potentials as the terrace width of the bismuth-modified surfaces becomes narrower, suggesting a diminution in the activation energy barrier as the ensemble size becomes smaller. Contrary to the behavior on the stepped surfaces, bismuth-modified, polyoriented spherical platinum electrodes, which contain a random distribution of (111), (100), and (110) faces, exhibit a continuous rise in catalytic activity for HCOOH oxidation with bismuth coverage, even at bismuth coveragesmore » approaching 100%.« less

Low imprint ferroelectric material for long retention memory and method of making the same

Abstract: A liquid precursor for forming a thin film (313) of ferroelectric metal oxides in an integrated circuit contains metal oxides in excess of the stoichiometrically balanced amount. When the precursor comprises strontium, bismuth, tantalum and niobium for forming strontium bismuth tantalum niobate, the precursor contains excess amounts of at least one of tantalum and niobium. Capacitors containing thin films (313) of layered superlattice material made from a precursor containing excess tantalum and niobium show good polarizability and low percentage imprint after 1010 negative polarization switching pulses at 75 °C, and after 109 negative polarization switching pulses at 125 °C.

Effect of thermal cycles on the mechanical strength of quad flat pack leads/Sn-3.5Ag-X (X e Bi and Cu) solder joints

Abstract: Quad Flat Pack (QFP) Leads/Sn-3.5Ag-X (X=Bi and Cu) joint was thermally cycled between 243 K and 403 K or 273 K and 373 K, and both metallographic examination and mechanical pull test were performed to evaluate thermal fatigue damage of the joint. The addition of bismuth drastically degrades the thermal fatigue resistance of Sn-3.5Ag solder. On the other hand, the pull strength of Sn-3.5Ag-Cu solder joints slightly decreased with increasing number of thermal cycles, though it still remains higher in comparison to that for conventional Sn-37Pb or bismuth containing solder joint. The behavior observed here reflects the isothermal fatigue properties of bulk solder, because thermal fatigue crack initiates at the surface of solder fillet and propagates within the fillet in an early stage of fatigue damage. Furthermore, the lead phases lying at the interface between lead-frame and bismuth containing solder joint may promote the crack propagation at the interface, resulting in the extremely low thermal fatigue resistance of the joint.

Ferroelectric Properties of Bismuth Layer-Structured Compound SrxBi4-xTi3-xTaxO12 (0≦x≦2)

Abstract: The ferroelectric and fatigue properties of bismuth layer-structured ferroelectrics (BLSF), SrxBi4-xTi3-xTaxO12 (x=0–2) [SBTT-x] with m=3, are studied for the purpose of obtaining large remanent polarization, Pr, and fatigue-free properties From the measurement of D-E hysteresis loops using a standard Sawyer-Tower circuit, the value of 2Pr for SBTT-03 ceramic was found to be the largest (2Pr=246 µC/cm2) in the SBTT-x system As a result of examining fatigue properties, SBTT-03 ceramic displays fatigue-free behavior up to about 1010 cycles It is assumed that these results are caused by the substitiution of Ta and Sr ions

Low temperature process for fabricating layered superlattice materials and making electronic devices including same

Abstract: A coating of liquid precursor containing a metal is applied (224) to a first electrode (122, 420), baked (226) on a hot plate in oxygen ambient at a temperature not exceeding 300 °C for five minutes, then RTP annealed (228) at 675 °C for 30 seconds. The coating is then annealed (230) in oxygen or nitrogen ambient at 700 °C for one hour to form a thin film (124, 422) of layered superlattice material with a thickness not exceeding 100 nm. A second electrode (126, 424) is applied to form a capacitor, and a second anneal (234) is performed in oxygen or nitrogen ambient at a temperature not exceeding 700 °C. If the material is strontium bismuth tantalate, the precursor contains u mole-equivalents of strontium, v mole-equivalents of bismuth, and w mole-equivalents of tantalum, where 0.8 ≤ u ≤ 1.0, 2.0 ≤ v ≤ 2.3, and 1.9 ≤ w ≤ 2.1.

Solubility of bismuth oxide in barium titanate

Abstract: The solubility of bismuth oxide in barium titanate was investigated by X-ray diffraction and scanning electron microscopy. Bismuth was added in two schemes: Ba1-xBixTiO3, where only site balance was considered, and Ba1-1.5yBiyTiO3, where both site balance and charge balance were considered. The solubility limits of bismuth in BaTiO3 sintered samples for these two schemes are slightly different, 3 and 5 at.%, respectively, at 1250°C.