C. J. Maggiore

Synthesis of molybdenum disilicide by mechanical alloying

Abstract Considerable interest and effort are being directed towards developing molybdenum disilicide (MoSi2) alloys with low oxygen content. During alloy synthesis, oxygen combines with Si to form glassy SiO2 precipitates at the MoSi2 grain boundaries, resulting in a degradation of its mechanical properties. We have used mechanical alloying, a high-energy ball-milling process, to synthesize alloy powders of MoSi2, MoSi2-27 mol.% MoSi3, MoSi2-50 mol.% Mo5Si3 and MoSi2-50 mol.% WSi2 starting from elemental powders. The processing of the powders, as well as the loading of the powders in graphite dies, was performed under high-purity argon inside a glovebox. The finer grain and particle size of the mechanically alloyed powders enabled us to hot-press them at 1500 °C, which is 300 °C lower than the temperature currently used for hot-pressing commercial powders. We have been successful in reducing the oxygen content in our alloys to about 310 ppm by weight, as measured by nuclear (d,p) reactions. We report the formation of metastable phases in the mechanically alloyed powders and their characterization by X-ray diffraction and differential thermal analysis. We also report the characterization of the hot-pressed alloys by optical and transmission electron microscopy, and the measurement of high-temperature mechanical properties.

Growth of (110)‐oriented CeO2 layers on (100) silicon substrates

CeO2 layers epitaxially grown on (100) silicon substrates by electron‐beam evaporation were investigated and proved to have (110) orientation. X‐ray diffraction measurements showed the CeO2 layers consist of more than 98% volume fraction of the (110) component. Cross‐sectional high‐resolution transmission electron microscopy and selected‐area electron diffraction clearly verified the above configuration of crystallographic orientations and that the 〈100〉 direction in the CeO2(110) plane is parallel with the 〈110〉 direction in the Si(100) plane. The cross‐sectional lattice image clarified the existence of a ∼60‐A‐thick intermediate amorphous layer between the CeO2 layer and the silicon substrate. Moreover, the high density of defects such as dislocations and low‐angle boundaries that exist in the vicinity of the interface agree well with Rutherford backscattering and channeling measurements.

a‐axis oriented YBa2Cu3O7−x thin films on Si with CeO2 buffer layers

We have grown a‐axis oriented YBa2Cu3O7−x (YBCO) thin films on Si(100) substrates with (110) oriented insulating buffer layers of cerium dioxide (CeO2) using the pulsed laser deposition technique. The films are highly oriented and textured as determined by θ–2θ x‐ray diffraction, x‐ray pole‐figure scan, scanning electron microscopy, Rutherford backscattering spectroscopy, and ion channeling. No diffusion at the interface has been found at growth temperatures up to 760 °C, indicating the CeO2 is a chemically stable and structurally compatible intermediate material for the growth of YBCO on Si. A zero resistance superconducting transition temperature of 87 K and a critical‐current density (Jc) of 1.5×105 A/cm2 at 75 K have been measured; Jc obtained represents the highest value for the a‐axis oriented YBCO films.

a -axis oriented YBa sub 2 Cu sub 3 O sub 7 minus x thin films on Si with CeO sub 2 buffer layers

We have grown a‐axis oriented YBa2Cu3O7−x (YBCO) thin films on Si(100) substrates with (110) oriented insulating buffer layers of cerium dioxide (CeO2) using the pulsed laser deposition technique. The films are highly oriented and textured as determined by θ–2θ x‐ray diffraction, x‐ray pole‐figure scan, scanning electron microscopy, Rutherford backscattering spectroscopy, and ion channeling. No diffusion at the interface has been found at growth temperatures up to 760 °C, indicating the CeO2 is a chemically stable and structurally compatible intermediate material for the growth of YBCO on Si. A zero resistance superconducting transition temperature of 87 K and a critical‐current density (Jc) of 1.5×105 A/cm2 at 75 K have been measured; Jc obtained represents the highest value for the a‐axis oriented YBCO films.

Alpha-, boron-, silicon- and iron-ion-induced current transients in low-capacitance silicon and GaAs diodes

High-speed current transient measurements were made over a large range of linear energy transfer (LET), using a wideband 70-GHz (6-ps-risetime) sampling oscilloscope on high resistivity GaAs diodes and 1-, 3-, and 10- Omega -cm silicon diodes. For 3- and 5-MeV alpha particles, 12-MeV boron, 18-MeV silicon, and 12- and 100-MeV iron ions incident on these devices, risetimes in the range from about 38 ps to 100 ps were produced depending on LET and device resistivity. Results are compared to the productions of various models. >

High‐energy elastic backscattering of helium ions for compositional analysis of high‐temperature superconductor thin films

A thin‐film technique for measuring the chemical composition of Y‐Ba‐Cu‐O thin films to a few percent accuracy is described. This technique utilizes non‐Rutherford backscattering of 8.8 MeV helium ions, which has an increased sensitivity 16O by a factor of 25 over Rutherford backscattering spectrometry. The ratios of the cross sections for He++ scattered from oxygen, copper, and yttrium relative to barium are easily determined using thin‐film standards that can be fabricated in any deposition system capable of producing thin‐film superconductors. The technique does not require the constant use of standards or accurate charge determination.

Ion beam channeling study on the damage accumulation in yttria-stabilized cubic zirconia

Abstract The lattice damage kinetics of defect accumulation in ion irradiated yttria-stabilized cubic zirconia (YSZ) was investigated by using in situ Rutherford backscattering spectrometry and channeling (RBS/C) techniques. The samples of single crystalline YSZ were irradiated with 400 keV Xe++ ions over a range of doses and for sample temperatures of 170 and 300 K. Under all irradiation conditions studied, the samples remained crystalline. However, lattice damage, as measured by the minimum yield, χmin, was observed to have three distinct stages (stages I–III) in the rate of accumulation. Energy dependent channeling experiments revealed −1 and 1 2 power dependence for the dechanneling parameter in stage I and stage III, respectively. Corresponding transmission electron microscopy (TEM) studies detected defects which appear as tiny dot contrasts in stage I samples, ultimately leading to the overlaps of dislocations at stage III. The three stages of damage accumulations have been interpreted as an evolving defect structure which changes the dominant RBS/C ion scattering process from direct scattering to dechanneling. A comparison between RBS/C and TEM techniques has also been made for the effective use of in situ RBS/C techniques for the study of radiation damage.

Orientation selection in thin platinum films on (001) MgO

Orientation selection in platinum films of ∼20 nm thickness deposited onto (001) MgO substrates by e‐beam evaporation was investigated through ion beam channeling and x‐ray diffraction. A mixture of crystallites having (111) and (001) orientation was observed in Pt films deposited over a range of substrate temperatures from 25 to 700 °C, with the (111) orientation dominant at low temperatures. The (111) orientation was present in these evaporated films at significantly higher substrate temperatures than reported for Pt films deposited by sputtering or pulsed laser deposition. Both orientations had strongly preferred in‐plane orientations: [110] Pt//[110] MgO for the (001)‐oriented crystallites and [110]Pt//[110] MgO for the (111). The orientation selection process was rationalized based on the expected relative interfacial energies for these two orientations.

Epitaxial growth of BaTiO3 thin films by plasma‐enhanced metalorganic chemical vapor deposition

High‐quality BaTiO3 thin films have been epitaxially grown on (001) LaAlO3 and (001) NdGaO3 substrates by plasma‐enhanced metalorganic chemical vapor deposition at a substrate temperature of 680 °C. X‐ray diffraction θ–2θ, ω, and φ scan results all indicate that single‐crystalline BaTiO3 thin films were epitaxially grown on the substrates with 〈100〉 orientation perpendicular to the substrates. The high degree of epitaxial crystallinity is further confirmed by Rutherford backscattering spectrometry which gives a minimum yield of 7.5% and 11% for films deposited on LaAlO3 and NdGaO3, respectively. Cross‐section high‐resolution electron microscopy images also showed that the layer epitaxy of BaTiO3 was characterized by an atomically abrupt film/substrate interface. Scanning electron micrographs showed that these films had very smooth surface morphologies.

Low‐loss microwave cavity using layered‐dielectric materials

Loss measurements have been carried out on a cylindrical pillbox cavity whose metallic flat end walls have been replaced by an ordered sequence of high purity sapphire and air dielectric layers. The loss of the TE01 mode at 18.99 GHz was substantially lowered. An improvement in cavity Q for this mode from 8.30×103 to 531×103 was observed. These experimental results closely reproduce two independent theoretical simulations. All measurements were taken at 30 °C.