P.K. Gallagher

Perovskite Oxides: Materials Science in Catalysis

In a time of growing need for catalysts, perovskites have been rediscovered as a family of catalysts of such great diversity that a broad spectrum of scientific disciplines have been brought to bear in their study and application. Because of the wide range of ions and valences which this simple structure can accommodate, the perovskites lend themselves to chemical tailoring. It is relatively simple to synthesize perovskites because of the flexibility of the structure to diverse chemistry. Many of the techniques of ceramic powder preparation are applicable to perovskite catalysts. In their own right, they are therefore of interest as a model system for the correlation of solid-state parameters and catalytic mechanisms. Such correlations [See figure in the PDF file] have recently been found between the rate and selectivity of oxidation-reduction reactions and the thermodynamic and electronic parameters of the solid. For commercial processes such as those mentioned in the introduction, perovskite catalysts have not yet proven to be practical. Much of the initial interest in these catalysts related to their use in automobile exhaust control. Current interest in this field centers on noble metalsubstituted perovskites resistant to S poisoning for single-bed, dual-bed, and three-way catalyst configurations. The formulations commercially tested to date have shown considerable promise, but long-term stability has not yet been achieved. A very large fraction of the elements that make up presently used commercial catalysts can be incorporated in the structure of perovskite oxides. Conversely, it is anticipated that perovskite oxides, appropriately formulated, will show catalytic activity for a large variety of chemical conversions. Even though this expectation is by no means a prediction of commercial success in the face of competition by existing catalyst systems, it makes these oxides attractive models in the study of catalytic chemical conversion. By appropriate formulation many desirable properties can be tailored, including the valence state of transition metal ions, the binding energy and diffusion of O in the lattice, the distance between active sites, and the magnetic and conductive properties of the solid. Only a very small fraction of possible perovskite formulations have been explored as catalysts. It is expected that further investigation will greatly expand the scope of perovskite catalysis, extend the understanding of solid-state parameters in catalysis, and contribute to the development of practical catalytic processes.

Oxygen stoichiometry in Ba2YCu3Ox

Abstract Thermogravimetry (TG), evolved gas analysis (EGA), x-ray powder diffraction and dilatometry have been used to characterize the oxygen stoichiometry in the 93K superconductor, Ba2YCu3Ox (7.0>x>6.0). Above 400°C equilibration with ambients containing oxygen occurs rapidly. A reversible structural transformation occurs with changing oxygen stoichiometry going from orthorhombic at x=7.0 to tetragonal at x=6.0. The upper limit of x, which is achieved by annealing in oxygen at 400–500°C, shows the sharpest superconducting Tcs. The tetragonal phase is not superconducting. Structural models are proposed for the tetragonal structure.

Water interaction with the superconducting YBa2Cu3O7 phase

We show that the superconducting YBa2Cu3O7 phase is highly sensitive to water and water vapor. This is probably due to the presence of nonequilibrium Cu3+ ions in this compound. In particular, the YBa2Cu3O7 phase decomposes in water to CuO, Ba(OH)2 and Y2BaCuO5 and evolves oxygen. Samples with a reduced oxygen content, e.g., YBa2Cu3O6.0, also decompose in an aqueous ambient. The superconductivity of YBa2Cu3O7 samples is greatly degraded by the interaction with water and humid air. This effect should not preclude practical application of these materials since it should be possible to protect them with coatings of metal, glass, or plastic.

The effects of sample size and heating rate on the kinetics of the thermal decomposition of CaCO3

Abstract Isothermal and dynamic methods were used to study the rate of weight loss of CaCO3. Sample sizes were controlled in the range of 1–32 mg. The contracting area rate law proved universally applicable. A pronounced dependence of the activation enthalpy, pre-exponential term, and rate constant upon sample weight and heating rate was observed. This dependence is discussed primarily in terms of the effects of self-cooling and sample geometry. The concept of a unique activation energy is questioned.

Thermal analysis of rare earth gallates and aluminates

Dilatometry, high-temperature x-ray diffraction, differential thermal analysis, and differential scanning calorimetry have been performed on LaGaO{sub 3}, NdGaO{sub 3}, PrGaO{sub 3}, SmAlO{sub 3}, and LaAlO{sub 3} single crystals grown by the Czochralski technique. First order phase transitions have been located at 145 {degree}C for LaGaO{sub 3} and 785 {degree}C for SmAlO{sub 3}, and {Delta}{ital H} has been measured for the LaGaO{sub 3} transition. Second order transitions have been identified for LaGaO{sub 3}, PrGaO{sub 3}, NdGaO{sub 3}, and LaAlO{sub 3}. The usefulness of these compounds as substrates for high temperature superconducting films is discussed in terms of thermal expansion matching.

Laser direct-write metallization in thin palladium acetate films

Micron‐scale palladium lines have been produced by a photothermal laser direct‐write process in thin palladium acetate films. The range of observed structures relates to the complex thermal profiles generated by coupling of the incident laser radiation with the exothermic heat of reaction. Surprisingly, the chemical composition of these features does not vary significantly as a function of laser power and scan speed. Rather, deviation of the electrical resistivities of these features from that of pure palladium results from porosity in the lines.

Characterization of LPCVD Aluminum for VLSI Processing

Aluminum and aluminum alloys are widely used for metallizing devices in VLSI processing. Such films can be deposited by a variety of techniques, which all presently suffer from inadequate step coverage. In this paper, we discuss the properties of aluminum films deposited by a low pressure chemical vapor deposition process using tri‐isobutyl aluminum as a source. Results of this work demonstrate that this process provides conformal step coverage, introduces no surface states, and promises to yield high wafer throughput. Films deposited on oxidized silicon monitors exhibit excellent properties in terms of chemical purity, adhesion, and electrical resistivity. Films deposited on device wafers prove to be compatible with current VLSI processing in terms of patterning, dry etching, and bondability and appear to have no effect on overall device performance. However, drawbacks of LPCVD aluminum appear to be in its structure‐related properties: namely, electromigration resistance and Al‐Si interdiffusion. These problems and potential solutions are addressed.

Kinetics of the thermal decomposition of CaCo3 in Co2 and some observations on the kinetic compensation effect

Abstract The kinetics of the thermal decomposition of CaCO 3 in CO 2 were investigated using both dynamic and isothermal techniques. Values of apparent activation energies range from 200–1000 kcal mol −1 depending upon sample size and heating rate. It is concluded that thermal transport rather than mass transport or chemical processes is rate determining. The results are compared with earlier work in O 2 and discussed in terms of “the kinetic compensation effect”, i.e., the reported linear relationship between the logarithm of the preexponential term and the activation energy derived from the Arrhenius equation.

Synthesis and properties of the YBa2Cu4O8 superconductor

Abstract A new simple synthesis route was used to produce the ‘124’ superconductor YBa 2 Cu 4 O 8 . The method utilizes YBa 2 Cu 3 O 7 powder (‘123’) as a precursor, which is then converted to the 124 superconductor by reaction with a stoichiometric amount of CuO through normal grinding and sintering, without the need for high oxygen pressure processing. Sintered bars of the 124 superconductor exhibited a T c of ∼ 75 K in resistivity and AC magnetic susceptibility measurements. Transport critical current density was measured to be ∼ 150 A/cm 2 at 60 K, and showed a strong field dependence. This behavior, in combination with a relatively high J c (magn.) of 4 × 10 4 A/cm 2 at 60 K and H = 0.9 T, is indicative of Josephson weak links at grain boundaries, which is similarly observed in the 123 phase. It is also noted that the intragrain J c in the twin-free 124 superconductor is about the same as that in the twinned 123 superconductor at ∼ 15 degrees below their respective T c .

Grain-growth enhancement in the YBa2Cu3O7−δ superconductor by silver-oxide doping

Abstract The addition of silver oxide has been found to accelerate the decomposition and melting of the Y-Ba-Cu-O superconductor. The doped sample exhibits, after sintering near 980°C, morphology of very large, stacked-plate grains for the YBa 2 Cu 3 O 7−δ phase and the presence of decomposition products. By contrast, doping with metallic silver does not noticeably induce such an effect. The enhanced grain growth in the Ag 2 O-doped superconductor is most likely to be the main cause of the previously reported increase in magnetization hysteresis and the superconductor suspension effect at 77 K. Beneficial effects of the addition of metallic silver such as enhanced oxygen diffusion and extremely low contact resistance are also discussed.