Martha L. Mecartney

Sol-gel synthesis of zirconia barrier coatings

A method for applying zirconia barrier coatings using a sol-gel method is described. The coatings of 8 wt % yttria-stabilized zirconia are applied by spin coating a solution containing zirconium alkoxides and yttrium acetate on to stainless steel substrates. Crystallization of the films was observed for thermal treatments in the range 750 to 1050° C. Excellent adhesion at the interface, due to significant coating-substrate interfacial reactions, indicates that this sol-gel route is a feasible method for applying zirconia coatings.

Grain boundary ionic conductivity of yttrium stabilized zirconia as a function of silica content and grain size

Abstract Impedance spectroscopy at temperatures from 350 to 700 °C and analytical electron microscopy were used to characterize grain boundary conductivity and grain boundary segregation of SiO 2 in 8 mol% yttrium stabilized zirconia (Y-CSZ). Colloidal silica in the amount of 1–10 wt.% (2.5–25 vol.%) was added as an intergranular phase. Various grain sizes were produced by sintering or annealing at temperatures of 1350–1600 °C for times from 0.1 to 100 h. The addition of intergranular SiO 2 led to decreased grain size (due to grain boundary pinning), increased grain boundary volume, and reduced total ionic conductivity. An increase in grain boundary width, from approximately 4 to 8 nm, was correlated with higher temperature anneal treatments and greater amounts of SiO 2 . High temperature anneals of samples with silica reduced the grain boundary volume fraction by increasing the grain size and restored much of the total conductivity lost by adding silica. The grain boundary specific conductivity and the total ionic conductivity were not significantly affected by SiO 2 additions less than 5 wt.% when samples of a similar grain size were compared. This led to the conclusion that limited use of intergranular additives to create superplastic Y-CSZ should not degrade the ionic conductivity for a specific engineered grain size.

In situ formation of superconducting YBa2Cu3O7−x thin films using pure ozone vapor oxidation

Superconducting YBa2Cu3O7−x thin films have been prepared by coevaporation using an ozone vapor jet as an oxygen source. Films exhibiting zero resistance at 82 K have been fabricated in situ under high vacuum conditions using substrate temperatures of 700 °C without a post‐evaporation anneal in oxygen. This process has implications for in situ measurements of fully superconducting surfaces using a variety of probes as well as for the fabrication of devices and structures whose properties are dependent on surfaces and interfaces.

Formation of colloidal silica particles from alkoxides

Abstract The growth of solution-derived monodisperse silica particles was studied by cryogenic transmission electron microscopy (cryo-TEM) in order to provide direct observation of the growth mechanism in colloidal particle formation from alkoxides. The silica particles were grown from tetraethoxysilane solutions using n -propanol and ethanol as solvents with an ammonia catalyst. A range of water concentrations was used. The reacting solutions were cryogenically fast-frozen at various times in the reaction and the samples were directly observed in the TEM. The results indicate that low density particles are seen initially, and these undergo a collapse to form the colloidally stable seed for further growth. Nuclear magnetic resonance was also used to follow the particle formation in order to test this growth model. Continued growth is postulated to occur by addition of small, low density particles to the growing seed, surmised from a lack of dense small particles and no evidence for aggregation of larger particles in the cryo-TEM results.

In situ growth of DyBa2Cu3O7−x thin films by molecular beam epitaxy

Films of DyBa2Cu3O7−x with transition temperatures as high as 89 K and with nominal thicknesses down to 35 A have been grown in situ using molecular beam epitaxy employing ozone as a source of reactive oxygen. The process has been successful with a variety of substrates including SrTiO3(100), SrTiO3(110), LaAlO3(100), MgO(100), and yttria‐stabilized zirconia. The films could be imaged with a scanning tunneling microscope at 4.2 K, indicating a conducting surface even at low temperatures.

Microstructural development and electrical properties of sol-gel prepared lead zirconate-titanate thin films

A systematic investigation of the microstructural evolution of fast fired, sol-gel derived Pb(Zr, Ti)O 3 films (Zr/Ti = 54/46) was performed by analytical transmission electron microscopy (TEM). It was found that the nucleation and growth of the sol-gel PZT films were influenced by the precursor chemistry. The precursor solution was composed of Pb 2-ethylhexanoate, Ti isopropoxide, and Zr n-propoxide in n-propanol. Porous and spherulitic perovskite grains nucleated and grew from a pyrochlore matrix for NH 4 OH-modified films, but no chemical segregation was found. These thin films consisted completely of porous spherulitic PZT grains (∼2 μm) when the firing temperature was increased. Chemical phase separation with regions of Zr-rich pyrochlore particles separated by Zr-deficient perovskite grains was observed in the initial stages of nucleation and growth for CH 3 COOH-modified PZT films. This phase separation is attributed to the effect of acetate ligands on the modification of molecular structure of the PZT precursor. Firing the acid-modified films at higher temperatures for long times resulted in porous perovskite grain structures. The residual porosity in these films is suggested to be a result of differential evaporation/condensation rates during the deposition process and the gas evolution at high temperatures due to trapped organics in the films. Dielectric and ferroelectric properties were correlated to the microstructure of the films. Lower dielectric constants (∼500) and higher coercive fields (∼65 kV/cm) were found for the acid-modified PZT films with phase separation in comparison to those measured from the sol-gel films with a uniform microstructure (∽ > 600, E c 2 .

In situ growth of DyBa sub 2 Cu sub 3 O sub 7 minus x thin films by molecular beam epitaxy

Films of DyBa2Cu3O7−x with transition temperatures as high as 89 K and with nominal thicknesses down to 35 A have been grown in situ using molecular beam epitaxy employing ozone as a source of reactive oxygen. The process has been successful with a variety of substrates including SrTiO3(100), SrTiO3(110), LaAlO3(100), MgO(100), and yttria‐stabilized zirconia. The films could be imaged with a scanning tunneling microscope at 4.2 K, indicating a conducting surface even at low temperatures.

Modeling the gelation of silicon alkoxides

Abstract This paper reviews the experimental evidence which suggests that the gelation of alkoxides can be modeled using analyses developed for organic condensation polymerizations, and presents a model which can be used to predict the gelation of acid catalyzed silicon alkoxides. Parameters that characterize the polymerization products, such as weight averaged molecular weight, are calculated from the extent of the molecular reactions using a recursive solution technique applied to the random branching theory of polymerization. The extent of the hydrolysis and condensation reactions as functions of time are calculated from the kinetic schemes of Assink and Kay [J. Non-Cryst. Solids 99 (1988) 359; 104 (1988) 112] and Pouxveil and Boilot [J. Non-Cryst. Solids 94 (1987) 347]. Results of this modeling give molecular weight versus conversion and molecular weight versus time. The predictions of gel times and conversion using this recursive technique combined with the kinetic models of Assink and Kay for TMOS are in reasonable agreement with the experimental data of Kelts and Armstrong [J. Mater. Res. 4 (1989) 423] if the effects of cyclization are considered. For the gelation of TEOS, using the kinetic scheme of Pouxveil and Boilot, the agreement is not as good due to the larger extent of cyclization.

Superplasticity in cubic yttria-stabilized zirconia with intergranular silica

Abstract The effect of amorphous silicate additions on grain growth and high-temperature deformation of 8 mol% cubic yttria stabilized zirconia was investigated. Fine-grained (0.5 μm) samples were produced by addition of 5 wt% colloidal silica. Dynamic grain growth was limited by the presence of this inert intergranular amorphous phase with low solubility for zirconia and yttria. Strain rates as high as 5×10 −3 s −1 at 1500 °C were observed under compression, similar to those observed in superplastic tetragonal yttria stabilized zirconia. Over 180% true strain (505% engineering strain) could be obtained within 1 h at 1500 °C. The stress exponent for deformation was calculated to vary from 1.3–1.7 at temperatures of 1300–1500 °C, respectively. Activation energies for superplastic deformation in the range of 340–410 kJ/mol were obtained for applied stresses of 10–45 MPa.

The direct observation of structural development during vanadium pentoxide gelation

The sequence of structural evolution in the gelation of vanadic acid to form vanadium pentoxide gels was studied using cryogenic transmission electron microscopy (cryo-TEM) and scanning tunneling microscopy (STM). Small whiskers form from initially homogeneous solutions, and then grow into crystalline ribbon-like colloidal particles. It is proposed that the whiskers form from polymerization of dioxovanadium cations. The ribbons then grow by continued addition of dioxovanadium cations which are supplied by the decomposition of decavanadate ions. In solution, the ribbon-like particles have dimensions of approximately 25 nm × 3 nm × over 1 μm. These ribbons are flexible perpendicular to the plane of the ribbon. Upon drying, a flat rigid mass of ribbon-like particles is formed. The ribbons examined by STM showed striations 3 nm wide, a value that corresponds with the width of the unit cell proposed by J. Legendre and J. Livage [J. Colloid. and Interf. Sci. 94 , 75 (1983)].